If you go into the largest building in Ulgoths, there is an exit to a lower level a huge room with pillars and a strange symbol on the floor and several fake doors, what am i suposed to do here?

Nothing yet. Wait and see after you've completed Durlag's Tower.

Nothing Is Here Until You Have Completed Durlags Tower.

Spoiler ~ May Spoil Things ;)
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This Is Where The Last Fight Of ToTSC Is Against A Huge Great Demon! There Is A Slight Puzzle To It, Not To Hard Tho!!

<font color="cyan">Binky, u didnt need to say out. Just say wait and see....cos he might never get it anyway..</font>

One thing I will say is you better get ready !!!!!!!!!!

<font color="cyan">I enjoyed the "encounter". Was fun!</font>