Hell Power is a 3 spell at Blood 2, requiring 3. The caster gets a huge buff consisting of:

• +2 to all magic paths they already have
• +4 attack
• +4 defense
• +4 precision
• +4 morale
• +2 magic resistance
• +4 protection

It has its drawbacks, though; aside from accruing 300 Fatigue, it provides a Horror Mark, and it allows Horrors to mob you during the battle. The Horror Mark is permanent, but the Horror-mobbing is for only the battle.

The chance for a horror to spawn is 20%, plus 2% per horror mark on the caster. The horror spawns in a random square in the 5x5 grid around the caster.

Since Hell Power requires 3 Blood Slaves as a level 3 spell, the caster must have at least 3 to cast it, and cannot overcast it (by spending extra Blood Slaves) for reduced Fatigue if they're just 3. Scripting your mage to cast Reinvigoration, which wipes Fatigue, won't work if the caster is unconscious (from the aforementioned Fatigue) when they're supposed to cast it. This makes the spell somewhat tough to use, requiring either 6 (or 5 and an extra Blood Slave) or preparatory setup with Communions to cast it without wasting turns. Extra levels in Blood are ideal, while Reinvigoration (the trait) and Encumbrance-less armor will let you resume doing stuff quicker if you don't have high levels or Slaves.

Of course, using Communion Slaves to cast Hell Power gives them the buff and the Horror-magnetism too; on the bright side, this makes them better at tolerating the spells of other Communion Masters in that battle, since the uneven path boost will reduce the Fatigue they get.