Payload

When Payload enters the battlefield, create a Payload token.
(It's a token battle with health 7 and "This artifact enters with 4 point counters on it")
Whenever a battle named Payload is dealt
lethal combat damage, instead, remove one point counter, and reset its health.
If there are no more point counters on it, destroy it, and conjure a card named Game Winning Explosion into your hand