I'am in the first level of underground tomb in graveyard district.
I can't open a door leading to chambers north of big cacoon of the spiders???
I used Farsight to gain info about the romms behind the door, but an exit leading to surface is blocked from the entrance in the level above (graveyard), it is next to Dark Elf Viconia???
Knock spell and pick lock of 125% wont work?

It is part of a later quest in Chap 4 or 5, so don't bother. Do the rest of the tomb.

This is not a later quest in chap four or five. This door is the last quest of chap three. It is the quest in which you must investigate the tombs for the leader of the Shadow Thieves in order to get info on where Iomen is being held.
You can initiate this quest by paying gaelen Bayle the 15,000 to meet the leader of the shadow thieve's. There are several quest he asks you to go on before he tells where Iomen is. This is the last.