Zellix, Sanity Flayer + Haunted One + Pith Driller + Ashnod's Altar + Intruder Alarm

Zellix, Sanity Flayer

Haunted One

Pith Driller

Ashnod's Altar

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 Ashnod's Altar 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 intricate engine relies on chaining creature loops to continuously churn out resources through the mana-generating power of Ashnod's Altar and the untapping chaos of Intruder Alarm. By sacrificing and recurring Pith Driller—using its own entering -1/-1 counter to strip away the +1/+1 counter granted by Haunted One's undying ability—you endlessly reset its entry trigger. Meanwhile, Zellix, Sanity Flayer leverages the milling process to populate your board with Horror tokens that benefit from Haunted One's power boost, feeding right back into the alarm system to sustain an infinite loop of death, mana, and milling.