1) Escape from the room where the sacrifice is taking place down the corridor to the side while the worshipers are preoccupied. You'll need the pentacle or a successful Luck roll depending on the options you choose in this room. Recommend the pentacle as you need a low luck score for the achievement. The pentacle can be found inside a book in the study.
2) Try door on left.
3) Fling door open.
4) Defeat the GREAT DANE.
5) You wish to escape: Test your Luck. For the achievement you must be Unlucky.
6) Slip in the pool of blood.