Compel Soul is a hex that lets Coen question the dead. It is the first spell he learns, taught by Anca after Laslea falls. It is an investigation tool rather than a weapon.
Cast it on a fresh corpse and the dead person replays the moments that led to their death. Quests use it to uncover what really happened.
Anca teaches it after the sacking of Laslea. She learned it from the convent library.
Keep mementos you find. An object tied to the dead person, like something they loved or the last thing they saw, makes the spell work better.