Compel Soul is the first Witchcraft hex Coen learns, from Anca, after Laslea is sacked. It makes the recently dead replay the moments before they died, which is how you crack many investigations.
Anca shows Coen the hex early in the story, after the attack on Laslea. You do not need to hunt down a manual for it.
Use it on fresh corpses to hear echoes of how they died. Carry a memento of the dead person when you can, because the spell reveals more with one.
Anca says she first tried the spell just to see if it was possible. She also says blood magic opens the caster to something dark, which ties into the visions Coen gets.
Image: Rebel Wolves / Bandai NamcoBlood Surge
Blood Surge is a night-only Vampirism ability that makes a target burst from the inside through their own blood. You unlock it by drinking Xanthe's blood, so it is tied to her questline.
Crimson Rush
Crimson Rush is a Vampirism power that makes your active abilities hit harder the healthier you are. It is open from the start, so you only need Corruption and skill points to learn it.
Mesmerise
Mesmerise is a Vampirism power that lets you take control of an enemy's mind. You unlock it with Lacra's blood during The Night of Horrors, so it comes from story progress.
Piercing Shriek
Piercing Shriek is a Vampirism power that stuns enemies with a banshee-like scream. You unlock it with Brencis's blood, either from a phial in Xanthe's Recruiters: Taste of Power or at the end of Our Rotten Roots.