Farrelite Priest

A mana filter with a fuse on it. The trick is the conversion: feed it generic mana, get white back, one-for-one, which on its face does nothing but recolor what you already have. The point is the toughness. A 1/3 body sits in front of early creatures while you launder colorless sources into the white you actually want to cast, and unlike a mana rock it can block. The four-activation governor is what stops it from becoming a free combo piece: chain the filter past three uses in a turn and the Priest hands you a death sentence on the next end step, so the card refuses to be the engine half of an infinite loop and insists on staying a creature that pays out at a sustainable clip. That is a remarkably careful piece of work for a set built around fast, breakable mechanics: most of its contemporaries either did nothing or did too much, and this one threads a deliberate ceiling into an otherwise mundane ability. The flavor of a cleric who can overexert himself unto exhaustion is doing real work too, encoding the fail-state into the fiction. It is small, it is white, and it never asked to be more than a fixing piece with a conscience, but the self-immolation clause reads like someone thinking hard about where an ability could break and quietly nailing the door shut.