Cauldron of Souls + Dross Scorpion + Arsenal Thresher + Altar of Dementia

Cauldron of Souls

Dross Scorpion

Arsenal Thresher

Altar of Dementia

How it works

1. Activate Cauldron of Souls by tapping it, giving Arsenal Thresher persist until end of turn.
2. Activate Altar of Dementia by sacrificing Arsenal Thresher.
3. When Arsenal Thresher dies, its persist ability and Dross Scorpion trigger.
4. Resolve the Dross Scorpion trigger, untapping Cauldron of Souls.
5. Resolve the Arsenal Thresher trigger, returning it from your graveyard to the battlefield with a -1/-1 counter on it, and then choosing to reveal at least one artifact card from your hand to remove the -1/-1 counter.
6. Resolve the Altar of Dementia ability, causing target player to mill cards equal to Arsenal Thresher's power.
7. Repeat.