Spells you cast from your graveyard cost less to cast.
Whenever one or more opponents lose life, you may cast target instant or sorcery card from your graveyard. If that spell would be put into your graveyard, exile it instead. Do this only once each turn.
Emet-Selch turns your graveyard into a second hand: cheap recursion plus the ability to flashback an instant or sorcery for free whenever an opponent loses life. You play a low-curve spellslinger/control shell, chip in damage or drain to trigger Emet-Selch every turn, and grind incremental advantage until you bury the table in value or assemble a loop. Cast removal and card draw from the bin, protect Emet-Selch, and convert recursion into inevitability.
Repeatable free spell recasts generate massive card advantage over a game
Cost reduction makes graveyard spells trivially cheap, fueling explosive turns
Dimir gives premium removal, counters, and card filtering to support the engine
Naturally synergizes with cheap drain and ping effects that constantly enable the trigger
Hard to grind out in attrition matchups since your graveyard refills your gameplan
Heavily reliant on the commander; repeated removal taxes you and stalls the engine
Legendary Creature — Elder Wizard
less to cast.