Aether Shockwave

The modal split here is the whole design conceit: a tapper that lets you pick which side of the Spirit divide goes to sleep, built for a block where Spirits were a faction you were either with or against. Choose to tap all Spirits and you have a one-sided Falter against the dominant tribe; choose to tap all non-Spirit creatures and you blank everyone else's non-Spirit blockers while your own Spirits swing in clean. The cleverness is that the card refuses to be symmetric on purpose: each mode is a sweeping tap that exempts exactly the half of the board you care about, so its value scales directly with how lopsided the matchup is along that one tribal axis. That is also its ceiling. At four mana for an effect that does not remove anything, only delays it for a turn cycle, the card lives entirely on the assumption that the metagame is sharply divided into Spirits and not-Spirits. Outside that frame, the second mode degrades into a glorified Sleep that hits your own non-Spirits too, and the first mode hits nothing at all. It is a tribal-war instant in the most literal sense: a button that only matters when the board is drawn along the line it was designed to exploit, and a dead card the moment it is not.