Invasion of Innistrad // Deluge of the Dead

Flash is the whole trick here. Most battles are sorcery-speed liabilities: they land on your main phase, and because a Siege enters under one opponent's guardianship while the whole table is invited to break it down, you end up racing to defeat your own permanent to unlock the transform. This one sidesteps the timing problem entirely. It arrives at instant speed with a targeted -13/-13, which turns it into an ambush removal spell that happens to leave a battle on the table afterward. Cast it on their end step or in response to a combat trick, wipe a creature (the -13/-13 is a toughness kill that dodges indestructible without ever saying "destroy"), and now you have a siege you are perfectly happy to dismantle yourself, since the reward on the flip is a grinding engine rather than a swing.

The two halves are priced against each other cleanly: a four-mana removal spell on the front would be a touch overcosted alone, so the design front-loads the value and leaves the back half incremental. Deluge of the Dead grinds rather than closes: two 2/2 Zombies on entry, then a repeatable ability that exiles a card from any graveyard and, when that card was a creature, hands you another Zombie. It is graveyard hate that pays you in bodies. One card doing removal, board presence, and graveyard interaction across two turns, with flash dictating exactly when the sequence begins.