Deathrender + Gravedigger + Eternal Witness + Altar of Dementia

Deathrender

Gravedigger

Eternal Witness

Altar of Dementia

How it works

1. Activate Altar of Dementia by sacrificing Eternal Witness.
2. Eternal Witness dies, triggering Deathrender, putting Gravedigger from your hand onto the battlefield and attaching Deathrender to it.
3. Gravedigger enters the battlefield, returning Eternal Witness from your graveyard to your hand.
4. Resolve the Altar of Dementia ability from step 1, causing target player to mill cards equal to Eternal Witness' power.
5. Activate Altar of Dementia by sacrificing Gravedigger.
6. Gravedigger dies, triggering Deathrender, putting Eternal Witness from your hand onto the battlefield and attaching Deathrender to it.
7. Eternal Witness enters the battlefield, returning Gravedigger from your graveyard to your hand.
8. Resolve the Altar of Dementia ability from step 5, causing target player to mill cards equal to Gravedigger's power.
9. Repeat.