Font of Agonies

The genius here is that the fuel is free, but only for a specific kind of deck: the trigger fires exclusively when you pay life. Not damage, not incidental life loss, but the deliberate spend baked into cards that offer life as a resource: painless activated abilities that ask a life payment, spells with a life kicker, planeswalkers and lands that let you trade life total for effect. The blood counters accrue as a byproduct of paying those costs, and once four have piled up, one and a black points a repeatable creature kill at whatever needs to die. That structure inverts the usual removal math. Most repeatable removal costs mana per use and asks nothing else; this asks almost no mana over the life of the card but demands a deck built around paying life on purpose. The four-counter threshold is what stops it from clearing the board the moment it resolves: you cannot destroy anything until you have already spent a meaningful chunk of your life total, so the enchantment rewards decks that treat life as currency rather than cushion. It is a removal source whose ceiling is set entirely by how much life the rest of the deck is willing to pay, and how eagerly it does so.