Revenge of the Rats

Black has a long lineage of spells that read your graveyard and hand back value, but most return specific cards or convert life totals; this one takes a raw census of dead creatures and pays it out in bodies. The flashback clause is what turns a single payoff into a two-stage plan: the first cast wants a yard already stocked, and the second asks you to keep churning between castings, since the count is a snapshot each time rather than a resource the spell consumes. The creature cards stay in the graveyard on resolution (only the spell itself is exiled by flashback), so the same yard feeds both halves as long as you refuel it, which rewards self-mill and sacrifice fodder that keep the number climbing across both casts. The tapped clause is the quiet governor on the swarm. Even as a sorcery, forced to resolve on your own turn, an untapped army of Rats would still ambush a return attack the following combat; entering tapped delays the board's usefulness by a turn, so the tokens arrive as future offense rather than an immediate wall or blockers. The ceiling here is entirely a function of how hard a deck can fill a graveyard with creatures, which is why the card is happiest in a shell built to grind rather than one hoping for a single explosive count.