Kinscaer Sentry

The attack trigger reads its X off the number of attackers when it resolves, which puts the reward exactly on the axis white weenie has always lived on: the wider your team is when this swings, the fatter the creature you can cheat out of hand and straight into the red zone. It fires once per attack, so this is not an engine that snowballs within a single combat; it is a burst of tempo, converting one card in hand into immediate damage by dropping it tapped and attacking, sidestepping summoning sickness and never routing through a defensive turn. On a developed board that single trigger can land something well above its natural curve point ahead of schedule, and because the count is taken before blockers matter, the value is settled on resolution. What limits it is how narrow the input is: creatures only, from hand only, put onto the battlefield rather than reanimated or ramped into. There is no route to blink a nonattacker into safety and no way to fish a body out of the yard. You are paid strictly for committing to the attack. First strike and lifelink on the 2/2 itself are the quieter half of the design, keeping it a real threat into open ground and buying a life cushion while the board builds, but the payoff is that one swing that turns a card in hand into an attacker that was never supposed to be here yet.