Black Sun's Twilight
The scaling here works on two clocks at once. As pure removal, the shell prices a creature's death by its toughness, the way Consume Spirit and its lineage of X-cost drain effects always have: you pay for exactly as much as you need to kill, no more. What sharpens the design is the threshold clause hiding inside that curve. Cross X to 5 and the spell stops being a one-for-one and starts refilling the board, dragging a creature of matching size back from your graveyard onto the battlefield tapped. That number is doing real balancing work: at five mana of X you are paying a genuine premium over rate-efficient removal, and the reanimation target is capped at that same value, so the card can never both kill the biggest thing on the table and reanimate a bomb that dwarfs it. The two halves are yoked to a single X, which means every point you spend is a decision about which axis matters more this turn: a smaller kill that leaves the graveyard clause dormant, or an overpayment that turns a removal spell into a two-for-one swing. That it all happens at instant speed is the quiet lever, letting you hold up interaction and then, in the same window, convert a defensive answer into a proactive rebuild the moment the math clears the threshold.
