Thornmantle Striker
The scaling runs entirely on tribal density, which puts this in the lineage of Elf payoffs that convert a wide board into a single burst: Elvish Champion made them bigger, Priest of Titania made them mana, and this one turns the count into removal or a reset. The modal split is the design choice worth sitting with. The counter-removal half is the strange one, a black creature that strips loyalty from a planeswalker, spore counters off a Thallid, or the counters off an opponent's slowly-fusing threat, and its usefulness swings wildly with the board it lands into. The -X/-X half is the reliable side, an enters-trigger kill spell whose size you already know before you cast it, no combat needed and no dedicated removal slot in the deck. Both halves share the same floor and the same ceiling: with a lone Elf on the battlefield, X is one and the trigger is a rounding error, which is why the body is priced as a fair midrange creature rather than a payoff. Cast into a developed Elf board, though, X climbs fast enough that one card answers a large threat outright while adding a 4/3 to the count for the next one. The card asks the deck to have already done its work; it pays off the width rather than creating it.
