In the House of Ashes #3 | Chapter 8 Dark Messiah Guide

Look around. In the barrack room you will find a Black Orb - this is the special key. Come back to the locked door in Library and use Black Orb. Go inside. In huge room are Necromancers. Kill them. This place looks terrified... The monsters confined to the wall... Open the door on the right side. This is the prison. The serfs are imprisoned there. But Necromancers have changed them into beasts, so you must kill them all...

The Library	The Black Orb -
Use the Black Orb to open the door.	The Necromancers!
Set up Fire Traps...	...and wait...
What the hell is this??!	Necromancer's laboratory... terrified...

Come back to the main chamber. There is undead Necromancer lying on the floor. He has Laboratory Trap Door Key, take it. Come back to the prison, take items and go on.

The Laboratory Trap Door Key	This is not useful, you already have better items than this.
He has resistance on everything.	Really terrified...
The blood is everywhere...	Another not useful item...
The corner behind the bed in one of the cells	Throw away the not useful items.