Scarlet Shield is a passive Vampirism power that heals you when you land a perfect block. You unlock it with Ambrus's blood, so plan your Ambrus quests around it.
Get Ambrus's blood from a phial or by draining him in The Gilded Gauntlet. Then learn it at a road shrine with a skill point and enough Corruption.
Once slotted it works on its own. Every perfect block heals you a little, so it rewards good timing.
Crimson Rush boosts your powers at high health, and Scarlet Shield helps keep you there.
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.
Compel Soul
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.
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.