Kelsien, the Plague

A pinger that pays itself is nothing new, but this one banks its dividend on the player rather than the permanent, and that single design choice reshapes the whole card. One damage to a creature you don't control rarely kills anything on its own, yet the death-trigger clause turns that ping into a bounty: soften a target, let combat or a second spell finish it, and the kill hands you an experience counter. Those counters are the payoff, because each one pushes the body +1/+1, so the removal plan and the clock are the same activation counted twice. Because experience counters sit on the player, they survive Kelsien dying and being recast: a table that keeps answering the assassin is still returning a larger threat each time, and while the ping itself is always exactly one, the creature swinging in keeps getting bigger. Haste means the tap ability is online the turn it enters, with no summoning-sickness delay on the removal work. Vigilance handles the other axis: attacking no longer taps it, so it can swing and hold up an activation in the same turn rather than choosing between beatdown and pinging. Two roles, value engine and win condition, folded into a three-mana body that scales in lethality precisely with how efficiently it has been killing things.