Lightning Runner

The energy economy is the whole gating mechanism here, and it is steep by design: two counters per attack, eight to cash in, which means four attack steps from this creature before the engine powers on once. Crucially, it generates energy on the attack declaration, not on combat damage, so chump-blocking it does nothing to slow the count; only removing it or keeping it home does. That math is the restriction paying for the payoff, because untapping all your creatures and tacking on an extra combat phase is the kind of effect that ends games the instant it fires. The trigger keys off this creature attacking, not your team broadly, so the path to the eighth pip is narrow: this one body has to keep swinging, turn after turn, while surviving the removal aimed at a five-mana 2/2 that does nothing until the loop comes online. Haste is the concession to that slowness, letting the clock start the turn it lands rather than asking you to babysit it through a rotation; double strike helps it close once the extra combats start chaining, less as evasion than as raw damage when the untap-and-attack-again loop gets going. It is an extra-turn effect rebuilt as a combat loop, with the ramp-up cost front-loaded onto a fragile attacker rather than onto the mana, and the loop only sustains if each new combat refills the energy faster than the next activation drains it.