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Conceptually, you cannot kill Demogorgon at all without invoking the scroll. In the game, you are given 1 full day (2 hours) to decide what to do with Demogorgon.  If you invoke the scroll and don't fight to kill him, he will remained imprisoned for another era, unable to go back to the Abyss or break free into Toril.  If you fight to kill him, you destroy his physical existence on Toril, sending him back to rule and plot in the Abyss.  If you can't or don't take any action in 1 full day, he will be able to leave his prison and wreak havoc on Toril.
By the lore in the game, Demogorgon is considered by many the most powerful superboss for Gorion's Ward.  But in terms of defeating him, he may or may not be that hard.  That's why there are mods trying to make him to be more in line with the in-game lore that he is supposed to possess immense powers.
Every party should be able to easily defeat Demogorgon on all difficulty settings up to Insane, as long as they actually fought through five levels of Watcher's Keep to reach Demogorgon.

The only trick is in the know-how. 

It is highly advisable that players make an effort to discover and develop their own strategies and tactics for defeating Demogorgon, otherwise the thrill from defeating a mightiest superboss in BG2EE could be totally spoiled. 
For those who do need advice or wish to learn how others defeat Demogorgon, here are four standardized strategies.

Strategy #1) Defeat Demogorgon before he shows up: The Thief Way []

Of course, we can't kill enemies before they show up, but that's exactly what we are trying to do here.  Any real thief (Jan, Yoshimo, Hexxat) will be able to access Spike Trap once they have acquired at least 3 million XP.  They can set as many as 7 Spike Traps on a single map.
Well, what does that tell us?  Right where Demogorgon will spawn, a sufficiently high level thief may set 7 Spike Traps in such a way that all the traps will fire on Demogorgon once he shows up hostile.  This way, Demogorgon can be instantly killed.  If not, try again.  Even if still not, you have at least gained an (unfair) advantage by significantly reducing his HPs before anything else starts.
However, no battle, no thrills. Though 100% legit, this is the least satisfactory and the cheesiest way to defeat him.  You have only succeeded in fighting him without actually fighting him.

Strategy #2)  Defeat Demogorgon in direct combat: The Divide-n-Conquer Way[]

After the opening dialog is over, Demogorgon will immediately gate in 2 mariliths to fight on his side.  If you kill both mariliths before defeating Demogorgon, he will gate in Glabrezus, at least 2 at a time, to fight on his side.  If you again kill the Glabrezus before defeating Demogorgon, he will gate in more Glabrezus, again at least 2 at a time.  He will keep doing so, whenever he sees you or your summons but not his Glabrezus.

So, what does that tell you?  Don't kill the mariliths!

As a matter of fact, the two ugly mariliths are no more than two hard-to-crack meatbags used by Demogorgon to distract you.  Having a durable summon to keep Demogorgon's full attention, you can "convince" the mariliths into chasing after you upstairs.  Having another summon to engage the mariliths upstairs, you can then conveniently park both of them at the entrance.  As long as the mariliths are kept alive, Demogorgon will not gate in more demons no matter what else you do.
From now on, the battle is tipped in your favor.  As long as you keep the fight downstairs, you will only need to worry about Demogorgon himself.

Demogorgon has three methods to hurt you in direct combat: 

  • disabling Gazes, 
  • Implosion (spell-like ability), and 
  • poisoning melee attacks. 
If you are strong enough, his Gazes rarely work on you.  Even on the rare occasions they do work, chances are you may get away with the disabling effects unscathed.
If you have both Stoneskin (Ironskin) and 100% fire resistance, his Implosion either does nothing or can only briefly disable you.
As for his poisoning melee attacks, if you play on Insane Difficulty, they do hefty damage and can destroy a Planetar as fast as in 1 round after the Planetar is summoned.  But you don't have to engage him in melee.  You may also choose to attack him from behind a tough summon, such as a Mordenkainen's Sword .  Demogorgon and other demons cannot hurt those animated swords with only melee attacks.  Additionally, Protection From Magical Weapons (either cast as a known spell or via a scroll) completely negates all melee attacks on you for 4 rounds per cast.

From the point of view of combat, you can technically treat Demogorgon as an enemy who combines a regular dragon's abilities (using Gaze as opposed to Breath) with a standard lich's immunities (modified or improved).

  • To open up a dragon for a spell-based assault, you lower his Magic Resistance first: the same strategy applies here (except that Demogorgon's MR can only be lowered for a few rounds and he is completely immune to the Lower Resistance spell, just like every lich).
  • To open up a lich for a weapon-based assault, you remove his combat protections first: the same strategy applies here.
  • You can't hurt a lich with Magical Missiles or Fireballs or Feeblemind or Lower Resistance or other similar lower-level spells: the same goes here.
  • But you can always hurt a dragon or a lich with ABHW (if not resisted): the same goes here.
  • You can always beat a dragon or a lich to death with a mighty weapon (if it hits): the same goes here.
  • During the time window when their combat protections are gone, you can quickly kill any dragon or lich as long as your party's weapon-based damage output is sufficiently high: the same goes here too. 

Fighting Demogorgon himself with a full player party (including one-member parties) is no more than a standard, straightforward boss fight.  It's a battle whose sole purpose is to see who will stand last.

Once Demogorgon is defeated, regroup and go upstairs to destroy the two meatbags you parked there earlier.

Strategy #3) Defeat Demogorgon in direct combat: The Good-Fight Way[]

It is hard to defeat Demogorgon if you choose to fight him in a good fight.

A good fight means you let each and every of your enemies have an equal opportuntiy to fight you and to kill you.

In this case, a good fight means: you kill the 2 mariliths as you should before defeating Demogorgon, and then try to fight Demogorgon while being constantly harassed by a horde of Glabrezus.
If you choose to fight a good fight, the biggest challenge is no longer Demogorgon himself, but the fact that a few Glabrezus in the beginning quickly turn into a big horde of Glabrezus, who can easily overwhelm you if you are not strong enough.
Here, the game AI works in such a way that everyone (including all the enemies, party members, summons and Demogorgon himself, unless they are under your direct control) frequently chooses to take a long detour based on their pathfinding logic.
In the confined space at the bottom of Watcher's Keep , Demogorgon is huge, Glabrezus are big, both stairways are narrow, and the overall space is limited.  It's easy to sense blocked unless taking a detour.  Because of that, Glabrezus may run upstairs while you are fighting Demogorgon downstairs, causing Demogorgon to immediately gate in more Glabrezus.  However, the situation may not change quickly, and the newly gated Glabrezus may also sense being blocked.  If they also try to run upstairs, Demogorgon will again gate in another batch.  After a while, there can be a big horde of Glabrezus angrily searching for you in the confined area.  As they tend to try to reach the same spot while taking a detour, they tend to converge in one or two places in the end.  Either they don't see you at all (this typically only lasts for a short time), or a horde will madly rush to ram over you.

Glabrezus move fast, hit hard, can't be fooled with invisibility.

Individually, they are not hard to defeat.  As a horde, they can ultterly destroy an unlucky Planetar in a single round.  How many player characters or companion warriors are more durable than a Planetar?

The worst case here: Gorion's Ward is totally surrounded by a horde of Glabrezus, blocked in every direction, cannot move even an inch away.

However, if you know what you are doing and your Gorion's Ward is a solo-capable character, you may very well still survive even if you seem to have been overwhelmed by the horde.  Protection From Magical Weapons will totally shield you from getting hurt by Glabrezus for 4 rounds.  Summoning a creature on top of yourself may nudge some Glabrezus away enough to briefly yield a path for you to free yourself.

To fight a good fight, you may really want to keep the initial few Glabrezus around Demogorgon alive.

Killing them or luring them away will only cause more coming in your way, which is pointless and can be frustrating.

Strategy #4) Fight Demogorgon without defeating him: The XP Farming Way[]

The fact that you may get a horde of Glabrezus chasing after you here works against you, if all you want is to defeat him in a good fight.  But it can also work in your favor, if you wish to farm XP here with a couple of hours of hard work. 

Each Glabrezus is worth 12,000 XP.

With 50+ kills, you get at least 12,000 * 50 = 600,000 XP.
Among the player community, getting free XP is typically loathed by many.  However, killing Glabrezus is no easy task, it's not free XP, it's hard work that deserves respect.  At the bottom of Watcher's Keep, you may reasonably expect up to about 1 million XP before Demogorgon permanently leaves the game.  If in TOB you wish Sarevok to become a mighty Fighter/Mage or Fighter/Thief before the end of TOB, you may find this is not a bad idea at all.  When doing so, reduce your party size as much as you can. 
If you have a strong party, fighting the horde of Glabrezus while intentionally keeping Demogorgon alive is not really hard.  You just have to avoid being overwhelmed. 

Interested?  Try it and see how it goes!

Additional Notes:

While all player parties with all sorts of characters can defeat Demogorgon in combat by making a little (extra) effort, the strongest class that can defeat Demogorgon has to be Wild Mage.  "Victory? Unclear. Destruction? Certain!" is their ultimate motto.  At high levels, they are extremely powerful, surpassing every other class or class combo in terms of destructive power.  For a straightforward fight like this one, they can just stand there casting each and every of their known spells that they think will work on Demogorgon, for (at least in theory) 100s or 1000s of times, one after the other without resting.  Technically, the only thing that can interrupt their spell cating is a Wild Surge that happens to have an effect that can interrupt their spell casting.
Demogorgon vs Dragon Disciple

A solo Sorcerer (Dragon Disciple) fighting Demogorgon for 2+ hours non-stop without leaving the area.

Here is a screenshot that shows a Sorcerer (Dragon Disciple) fighting Demogorgon for over two hours non-stop, without leaving the area.  As you can see, Demogorgon has moved upstairs along the upper stairway.  Because he blocked the upper stairway while moving on it, some of the demons took the lower stairway.  At this specific moment, it was a battle of 6 vs 8.  Not too bad. Demogorgon had been badly injured but kept alive.  For the moment he was gaining an upper hand by overwhelming the Sorcerer with a horde of fast-moving hard-hitting demons.
The screenshot illutrates the scale of the fight.  It may be hard or very hard to win, but it's not a really big fight.

Screenshots of A Full-Scale Battle[]

I finally got a few screenshots of a full-scale battle with Demogorgon to complement the previous screenshot from one of my solo playthroughs.  This time, I planned to fight Demogorgon for only an hour or so, but once the battle escalated to a full scale, I found I had to spend much more time trying to work out a safe way to defeat him.  In the end, each party member earned 447,168 XP.  The total XP for the party = 447,168 * 6 = 2.683 million.  Excluding 23,000 XP for killing a Marilith (the other one didn't award XP), 100,000 quest XP for defeating Demogorgon, and 20,000 XP for talking to Odren, the total XP for killing Glabrezus = (447,168 - 100,000) * 6 - 23,000 - 20,000 = 2.040 million.  Dividing the number by 12,000 yields 170, which is the number of the Glabrezus killed, excluding a few kills that did not award XP.
For the record, Viconia leveled up 3 times, Neera and Sarevok each leveled up twice, no party member took damage, and the game was only played on Insane difficulty with 100% extra damage taken (On this setting, Implosion does up to 400 damage per strike, bypassing MR, if targeted at whoever is in my party).
1

Demogorgon with 22 minions fighting player party of 6 with 4 summons

Here is a screenshot showing the scale that the battle escalated to.  At the moment, Demogorgon was being helped by 22 demon minions, while my Shaman's 6-member party was being helped by 4 summons.  It was his party of 23 vs my party of 10. Ha!!!
20+ demons might look scary, but they were not hard to destroy.  A small team (Neera + Sarevok) destroyed most of the 20+ demons shortly, leaving only a lonely Demogorgon behind.
Once the battle escalated to a full scale, the biggest trouble is that as documented on this wiki page, Demogorgon will restore his Magic Resistance and also heal himself every time he gates in more minions. 
4

Demogorgon forced player party to relocate downstairs

Demogorgon eventually captured my party's base on the upper platform, forcing the party to retreat downstairs.
I made three attempts to end the battle.  The first two attempts relied on Neera + Jan .  Although they killed a huge number of demons, they were too slow to prevent Demogorgon from recovering quickly.  In the end, I decided to transfer Robe of Vecne from Aerie to Neera before making my 3rd attempt with Neera + Aerie.  Aerie's Project Image attacked Demogorgon (who was on the upper platform at the time) from the right-side stairway, assisted by Viconia.  Neera's Project Image attacked Demogorgon from the left-side stairway, assisted by Sarevok and my Shaman.  Jan stayed downstairs just in case we would have to retreat.  When the two Project Images expired, Demogorgon was "badly injured" but was fast recovering.  To ensure his defeat, I left two floating swords on the upper platform to keep a number of demons in his sight.  Meanwhile, Sarevok and my Shaman cleared a path to him from below along the left-side stairway.
6

Demogorgon was about to acknowledge his defeat in disbelief

Here is a screenshot right before the cutscene was played showing Demogorgon acknowledging his defeat in disbelief.  Neera's 2nd Project Image "killed" him before he could gate in another batch of minions to heal himself again.
HenryNY (talk) 11:26, March 30, 2020 (UTC)

A Few More Tips[]

Here are a few tips if you wish to keep Demogorgon alive for at least a while in combat:
1) Try to block one of the two stairways, if you wish to earn extra XP for fighting him.  Whenever his minions go for the other stairway, ambush and kill them out of his sight so that he will keep gating in more for you to kill.
2) Try to block both stairways, if you wish to end the battle by defeating him.  If you need to get close to him for using Whirlwind on him (or for whatever other reasons), clear a path only from one side, keeping demons on his other side alive and busy.
3) Don't block the stairways, if you wish to disengage from him.  Kill whoever follows your party and then you may make a (quick) save or take a break.
4) You will have to use the Wish spell for its resting option, if you wish to fight him for as long a time as possible.  If you don't rest, your party will sooner or later be crippled by accumulated fatigue.  If you use the "rest for 8 hours" option, you may very well wake up only to see a message saying that Demogorgon has set himself free and permanently left.
5) The best tank for this battle are Mordenkainen's Sword s.  Keep them as few as you can, and try to spread them out so they don't get dispelled (insta-killed) at the same time.
I don't know your protagonist or party.  But I do believe that all the player parties who have fought through Watcher's Keep to get here are capable of defeating Demogorgon in combat fair and square.  If there is a trick, it is the know-how.
Have fun!
HenryNY (talk) 11:26, March 30, 2020 (UTC)

Solo the Prince of Demons[]

This method was designed with the principles of speed and diversion. Being the aggro of Demogorgon is dangerous and one should always try to kill him as fast as possible, as he hits very hard, has serious disables, high regeneration, strong defenses, and can summon demons to top it off. Therefore, having some decoy, even if short lived, is essential. This method was tested with a Cavalier solo (+3 to hit and damage against him). For this strategy, you will need some instances of Greater Whirlwind, Summon Deva, Carsomyr and the Helm of Vhailor.

Use the Helm of Vhailor to cast Simulacrum. Summon a Deva if you wish, with the clone or the original. Draw Upon Holy Might with you and the clone, and protect from evil yourself, the clone and the Deva. Use Greater Whirlwind with the clone and yourself and have Demogorgon appear. Once he is summoned, show that baboon headed who is the real boss of this realm, hack and slash him and only him. Pay no mind to the other demons around. The Deva will aggro some of them or even Demogorgon and will buy you some precious seconds of free hits. Carsomyr will constantly disable his defenses, so reuse Greater Whirlwind as needed. Mind, however, that even after Demogoron's body is destroyed, you need to deal with the hasted demons he summoned. The fight should last 10-20 seconds if handled properly. Avoid dragging beyond that, as Demogorgon and his demons hit hard and you need the Simulacrum and yourself hitting him constantly. The enemies are very deadly, so several retries may be necessary.

After being successful, write your name in history as the most powerful mortal ever, able to kill the Prince of Demons all by yourself. -- Added by a FANDOM user (talk)

NOTE*** Whatever strategy you use, Pierce Magic will make things much easier. The initial protections he casts at onset of fight can be debuffed, which is basically a necessity in this fight. -- Added by a FANDOM user (talk) 21:32, December 30, 2019 (UTC)
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