Zellix, Sanity Flayer + Haunted One + Pith Driller + Krark-Clan Ironworks + Intruder Alarm

Zellix, Sanity Flayer

Haunted One

Pith Driller

Krark-Clan Ironworks

Intruder Alarm

How it works

1. Activate Zellix by paying 1 and tapping it.
2. Haunted One triggers, causing all Horrors you control to get +2/+0 and undying until end of turn.
3. Resolve the Zellix ability, causing a player to mill three cards.
4. If the player milled a creature card this way, Zellix triggers, creating a 1/1 Horror creature token.
5. Activate Krark-Clan Ironworks by sacrificing Pith Driller, adding CC.
6. When Pith Driller dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
7. When Pith Driller enters the battlefield, it and Intruder Alarm trigger.
8. Resolve the Pith Driller trigger, putting a -1/-1 counter on it.
9. Resolve the Intruder Alarm trigger, untapping all creatures.
10. Repeat.

This crafty loop relies on Zellix, Sanity Flayer as your commander to trigger Haunted One's undying grant simply by tapping, while Krark-Clan Ironworks converts your sacrificed artifacts into the colorless mana needed to fuel the activation. When Pith Driller enters the battlefield, its own drawback of placing a -1/-1 counter on itself actually becomes an asset, neatly resetting any +1/+1 counters accumulated from its repeated undying returns so it can keep dying indefinitely. Meanwhile, Intruder Alarm ensures that every single creature entry refreshes your board by untapping Zellix and your mana producers, turning mill triggers and creature tokens into an unstoppable engine.