When emerging from the underdark you meet a clan which attacks you and so you kill them. You also get the impure water, and spoil the pure one so you kill the shadow prince. And so you get his blood. What it it for?

You need it if you do the Drow city quests.

Spoiler
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It's for the second or third to last Drow quest. The matron Mother needs it to summon A demon. The one you did was the easiest. The hardest is the elder orb and his buddy. (The mind flayer lair is almost as tough)!
I think the Underdark is the hardest chapter in the Game!

I do not mean the elder barin liquid, but the kuo toan prince blood, after I leave the drow city and while going to the surface.

It's the same quest, you have 3 options to use.

Kua Toan prince blood (which you found on the way out of the Underdark).
Elder Brain Liquid stuff
and whatever the Mind Flayer crap is.

You just happend to find 1 of the options on the way out, if you already did that quest, the blood is useless.

Elder brain eyestalk, btw [img]tongue.gif[/img]

I havent played BG2 in over a year and a half, sue me [img]tongue.gif[/img]

Names dont come clearly, much less where it came from :(

(Yet another Spoiler)
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That group of adventutrers that attacks you, are one of the few *good* people that youre forced to kill in the self defence, you need the Kuo-Toan Prince blood for the Matron mother so she could summon a demon, Going to the beholder lair is the easiest option you could have chosen there because the first thing you encounter is the elder orb. after killing it you can return to the matron mother, though I clean sweeped The Illithid Lair,Beholder City & Kuo-Toans for the goodies