Whenever a Horror you control deals combat damage to a player, that player mills that many cards.
At the beginning of your end step, choose target artifact or creature card in an opponent's graveyard that was put there from their library this turn. Put it onto the battlefield under your control.
Assemble a board of Horrors with built-in menace, swing to mill opponents for big chunks, then steal the best artifacts and creatures their library dumps into the graveyard during your end step. You grind value off opponents' decks while building a snowballing board, eventually winning through evasive combat damage or sheer accumulated theft.
Repeatable, asymmetric reanimation that steals from opponents rather than spending your own resources
Menace on all your Horrors makes combat damage (and triggers) hard to block
Mill doubles as a wincon and as fuel for N'ghathrod's end-step theft
Dimir gives access to the best card draw, control, and tutoring in the format
Punishes graveyard-reliant opponents while turning their cards against them
The steal trigger only hits cards milled that turn, so it whiffs against decks with few targets or if you can't connect
Horror tribal is shallow, forcing you to lean on changelings and token-makers
Commander-dependent: removing N'ghathrod kills your menace anthem and theft engine
Slow to close if blockers stall your attacks before menace matters
Vulnerable to graveyard hate and to opponents who exile their own milled cards
Makes a Horror token attacker with evasion and recursion to keep the mill engine humming.
Add changelings and Horror-makers (Maskwood Nexus, Realmwalker, Universal Automaton, Mistform Ultimus) to deepen the tribe and ensure every body carries menace. Lean into mill-payoff and evasion enablers plus cheap interaction to protect N'ghathrod, and consider self-mill-into-reanimation backups so you aren't fully dependent on opponents having juicy targets. Tighten the mana base with Dimir duals, fast mana like Fellwar Stone and Arcane Signet, and tutors such as Vampiric Tutor to find your engine pieces reliably.