Mortuary + Altar of Dementia + Crimson Kobolds + Bolas's Citadel

Mortuary

Altar of Dementia

Crimson Kobolds

Bolas's Citadel

How it works

1. Activate Altar of Dementia by sacrificing Crimson Kobolds
2. When Crimson Kobolds dies, Mortuary triggers, putting Crimson Kobolds from your graveyard onto the top of your library.
3. Resolve the Altar of Dementia ability.
4. Cast Crimson Kobolds from the top of your library by paying 0 life.
5. Repeat.

This loop relies on Crimson Kobolds as a zero-mana creature that can be repeatedly sacrificed to Altar of Dementia to mill an opponent out entirely. Because Crimson Kobolds costs zero, you bypass the usual life payment required by Bolas's Citadel to cast it from the top of your library. Meanwhile, Mortuary acts as the glue that makes the whole engine infinite, intercepting the Kobolds as they die and instantly sliding them right back onto the top of your library so Bolas's Citadel can play them again.