First of, do you have a Paladin? If yes, find the Medallion of the Lost Followers in the room to the south-east. Go back to Kuldahar graveyard and ... Good Luck.
Failing or succeeding that, you need to put the poison into the four basins. Noone tells you this, you're supposed to figure it out. Remember the stench from the hydraulic door? That's the poison. Happy rewinding!...
You'll get it soon enough.