Fervour is your engine
Fervour powers your Prayers and certain Mea Culpa abilities. You build it by fighting well — landing hits fills the bar, and executing a stunned enemy grants a flat 9.98 Fervour, the single biggest chunk per action.
Prayers run on fervour
Every Prayer has a Fervour cost between 20 and 100. Següiriya to your Eyes like Stars costs 20, Debla of the Lights 40, Soleá of Excommunication 60, and Aubade of the Nameless Guardian 100. A bar that stays near full lets you cast the answer to any fight — which is exactly when a boss from the boss database is bearing down.
Blood Penitence
You can also convert health into Fervour directly: Blood Penitence sacrifices 15 HP and 25 (plus 25 per sword level) Tears of Atonement for 25 Fervour. It is a panic button when the bar is empty and a Prayer would save the run.
Build it through executions
The fastest, safest Fervour comes from executions. Stun an enemy from the enemy database, execute it, and the meter climbs while you also bank 50% extra Tears. Targets like the Soldier of the Anointed Legion (500 Tears) or Sepulcro Insomne (500 Tears) pay double.
Spend it on purpose
Do not hoard Fervour. Spend it on the Prayer that answers the fight — a boss like Quirce (2,700 HP) or Crisanta (2,900 HP) is exactly when your abilities should be online.