Senu, Keen-Eyed Protector

The tap ability reads like a defensive out (gain some life, dig two cards deep) but it is really a positioning trick: exiling this Bird is the setup, and the payoff is the last line of text. Send any legendary creature you control into an unblocked attack and Senu drops back in already attacking, a second flier arriving from a zone the defender cannot interact with in the combat step. The engine rewards a board built around legendaries, which is precisely the deckbuilding pattern this shell is built to reward, so the self-exile is not a sacrifice; it is a loaded spring. What makes the loop honest is that the reentry demands a connecting attacker and an open lane: no unblocked legend, no return, and the 2/1 body stays in exile doing nothing. Flying and vigilance keep the first cast from being a one-and-done, letting Senu attack and still hold the tap ability in reserve, which is the timing wrinkle: you choose the turn to exile based on the board rather than being forced into it. It is a small creature doing structural work, converting a life-gain button into a repeatable ambush that fires off the wider board's aggression rather than its own.