Sandman's Quicksand
A symmetrical sweeper that becomes one-sided if you route it through the graveyard first: cast from hand, it clips both boards for -2/-2; discard it and pay the mayhem cost, and the same effect spares your creatures entirely. That conditional is the whole design axis. A flat -2/-2 to all creatures is a fair, unremarkable rate for a board where you have skin in the game, the kind of edge-cutting wrath that has always demanded you build lean enough to survive your own spell. Mayhem rewrites the deal by asking a different question: can you get this card into your graveyard on the turn you want it, through a discard outlet or a rummaging effect, rather than casting it from hand. When you can, you turn a mutual concession into a clean asymmetric edge, and the extra mana you pay for the mayhem cost is the toll on that upgrade. The card is built for a shell that treats the graveyard as a second hand, where discarding a sweeper is not a cost but a setup. It functions as an ordinary two-sided answer for the deck that stumbles into it, and as a lopsided one for the deck that plans around the discard. The gap between those two modes, and how deliberately you can widen it, is what separates the pilots who get value from this from the ones who merely get a Bane's board wipe.
Rules text
(You may cast this card from your graveyard for if you discarded it this turn. Timing rules still apply.)
All creatures get -2/-2 until end of turn. If this spell's mayhem cost was paid, creatures your opponents control get -2/-2 until end of turn instead.