After I have finished Durlags tower I go back to Ulgoths beard, my dagger gets stolen so i talk to the guy, he tells me to go and stop the summoning. I go to the summoning I dont stop it but I kill all the people, exsept for the Tana'ri because it wont die where ever I kill it, it just comes back. One time I finally killed it but I never got any XP and I did'nt get the dagger what do i do

You should first talk to the cultist in front of the stairs. Then she releases the beast and you can:

1.- Kill the beast, and he will take the place of a cultist, so this way you must defeat it about 10 times (very hard)

2.- Kill first the cultists and then deafeat AecLetec, and you will get 15 000 xp and no dagger, but talk to the dwarf and he´ll give you xp and a item (not very good item by the way)