In the lower level of the tomb at the start of SoD, you'll find a set of colour-coded torches in a room crawling with undead. One sconce has an unlit torch. Grab it, head over to the purple cauldron, and interact with it. Go back and put the lit torch in the sconce. A secret area will open up. Examine the desk in that room. Inside is a note by Sarevok. Pick it up to unlock the achievement.