Long River Lurker

An enters-triggered flicker packaged onto a cheap blue Frog, with an evasion enabler stapled on to make sure it fires. When it arrives, one of your creatures becomes unblockable for the turn, and if that creature connects, you may exile and return it. The payoff is the reset: returning a creature you control re-triggers every enter-the-battlefield ability it carries, so a single unblocked swing becomes a blink of a value engine's front half. The flicker is welded to combat damage, which is the restriction that pays for the loop: you have to land the hit first, and the unblockable grant is what makes landing it reliable. That same welding is why this is not a defensive trick. The flicker only fires on damage, so it cannot be held up at instant speed to save a creature from a removal spell; the reward comes during the combat damage step, not in response to a threat on the stack, and the whole package expires at end of turn. The Frog tribal line is the quieter piece: this creature protects itself with its own ward, and hands the same tax to your other Frogs, guarding the wider board rather than the Lurker alone. On an empty battlefield it is a 2/3 with a self-protection tax. Give it a creature whose arrival matters (a value ETB creature, a sacrifice payoff, anything whose ETB you want to see twice) and one swing turns into two triggers.