Soul Burn

Pour red mana into X and the spell hits harder; spend black mana and you drink the damage back as life. That single mana constraint is the design's load-bearing fence: every life point you recover is capped by the you spent, so leaning on red for reach means leaning away from the lifegain. A player casting it is choosing how black-heavy to build the spell, trading raw burn against the size of the life swing. The other three caps (you cannot gain past the target's toughness, a player's life total, or a planeswalker's loyalty before damage) close the door on the obvious abuse: a naive lifelink-on-burn effect manufactures life out of overkill, hitting a small creature for ten and gaining ten. Here the refund is fenced on four sides so the lifegain always tracks something real rather than the size of the number you paid for. The restriction-stacking is meticulous, the work of designers still mapping how much value one removal spell could fold in before it tipped. The verdict baked into the rules text was generous: a lot of reach and a lot of lifegain, provided your manabase earns both halves at once.