Kangaxx

Kangaxx is a powerful demi-lich whose body has been broken into pieces past and hidden around the city of Athkatla by his enemies. Restoring his body eventually results in acquiring one of the best rings in the game.

Kangaxx the Lich
When Kangaxx first arises from his tomb, he is a fairly standard lich, albeit more powerful than a standard lich, or even the guardians of the parts of his body. He wears the powerful Ring of Gaxx, granting him rapid regeneration, bonuses to his armor class and saving throws, and adding to his innate magic resistance. (It also gives him immunity to poison and disease, but since he's undead, those aren't really a concern for him.)

It is very important for the party that at least one person not be protected from undead, because once Kangaxx is down to 1 HP, he stops fighting and speaks to the party, revealing that he is in fact...

Kangaxx the Demilich
Once Kangaxx reveals his demilich form (which is essentially just a floating skull), the party is in for a difficult fight. Kangaxx the Demilich is 90% resistant to all physical damage, and like adamantite golems he can't be hit by anything with less than a +3 enchantment. On top of that, his saving throws are all 2 steps better than his lich form... except save vs. spell, which is 3 steps worse.

Since Kangaxx is (somehow) still wearing the Ring of Gaxx, he regenerates at a phenomenal rate.