I have cleared three levels of Dragon's Eye, extracted the poison, dispatched the "little boy" seeking his Mama. I could put the poison in the water four basins, but nothing has told my characters to do this. I think I have extracted all the information I am going to get from Nheero Fhutma. The Alchemist is still there but seems useless. The only unfinished task is the hydraulically operated door that I can't open.

What do I do next?

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!... :D You'll get it soon enough.