Whenever a card is put into an opponent's graveyard from anywhere, put a +1/+1 counter on The Haunt of Hightower.
The Haunt of Hightower is a mono-black voltron commander that grows enormous by exploiting your opponents' graveyards. You force opponents to mill, discard, and sacrifice cards, then turn the resulting counters into a flying, lifelinking beater that closes games in a couple of swings. Protect it with evasion-supporting equipment and recursion, then connect for lethal commander damage.
Snowballs to huge sizes fast since counters trigger off any card entering any opponent's graveyard
Built-in evasion (flying) plus lifelink stabilizes your life total while attacking
Cheap to recast and easy to support in mono-black with strong tutors and recursion
Doubles as removal and disruption via attack-trigger discard and mill effects
Classic voltron fragility — one removal spell or board wipe resets all your investment
Mono-black struggles with enchantments and has limited ways to deal with fliers/blockers it can't kill
Slow to come online without enablers; a 6/6 by itself dies to chump blocks and edicts
Reliant on opponents filling graveyards, which graveyard hate or low-interaction tables can blunt
Turns all the graveyard fueling into direct damage, giving a backup win and counter triggers.
Add more graveyard-filling payoffs and protection: trample enablers like Whispersilk Cloak or Loxodon Warhammer ensure damage gets through, while reanimation (Reanimate, Animate Dead) and counterspell-style protection (Imp's Mischief, Heroic Intervention is off-color so lean on Swiftfoot Boots) keep the threat sticky. Tighten the curve with fast mana (Dark Ritual, Sol Ring) and efficient mill/discard so the Haunt is lethal-sized by turn 5-6. For higher power, include a tutor package to find protection on demand and a backup wincon like Syr Konrad to avoid being purely voltron-dependent.