Once you have accepted Lavallin's quest to find his stolen things, you'll chase the thief and find him in a distant warren where you meet Bartlemy. Bartlemy will ask you to retrieve his ring from Lavallin's store. He will give you a wardstone which will lead you to sewers. In the sewers there are several encounters, one of them is with smugglers selling rithellium weapons, and another one is with a mage bying these weapons. Graeme or the mage drops the key to the hidden exit (stairs) which leads directly to senior staff appartments. From there you may return to the major hall and enter Lavallin's store again. So this is basically the way between Lavallin and Bartlemy.

This quest messes with quest to recieve Calinor body - if first recieve this quest you will go from halls to sewers and come to locked door, but the key is from other side. Sio there are some misunderstanding. Also after going to crypt and battle with lich there are no way out.

And nobody said it would be easy.

Seriously, if you decide to loot the coffins in the crypt magically protected then prepare for surprises.