This is a part of the Silver Sword. The blade is introduced during Baldur's Gate II Chapter 4.
Description[]
| “ | This is a piece of Silver Sword, a vorpal two-handed sword used by the githyanki and considered holy by them. It is magically sharp and can sever limbs and heads with ease. If the whole weapon could be found, it could be magically reforged. | ” |
Aquisition[]
There are several ways this blade will or can be obtained by the party.
The first way is to kill or pickpocket Saemon Havarian as part of the Escaping the asylum questline in the Spellhold Basement, or even later in the Spellhold Main Floor.
Saemon Havarian will also insist that the party takes the blade as a reward if the party participated in the Leaving the island questline, as a prize for helping him steal the Pirate Lord Desharik's personal vessel.
This method is probably best as it allows the party a chance to take part in the Underwater City questline. If the party opts to take the portal in the Spellhold Basement - they will miss out on that entire quest and adventuring area and hundreds of thousands of XP and treasure.
If you pickpocket the blade, oddly enough Saemon will still give you another duplicate blade if you choose to leave Brynnlaw by ship.
Unlike other scripts where it is removed from his inventory and transferred to yours, a second blade is created (which is a coding oversite - not intended for plot purposes).
Having a second blade allows some actions that are probably not what was planned by the developers - but in the end the party can still make only one Silver Sword if both components are brought to Cromwell.
Having an extra silver blade allows the party to be kinder to the githyanki who are hunting their blade in chapter 6. The party can return it and avoid bloodshed if they desire (and then of course, still be able to get the Silver Sword reforged, if the Silver Hilt is pickpocketed from Kruin the githyanki). Roleplay this as you will.