Prowling Geistcatcher + Phyrexian Delver + Gray Merchant of Asphodel + Altar of Dementia

Prowling Geistcatcher

Phyrexian Delver

Gray Merchant of Asphodel

Altar of Dementia

How it works

1. Activate Altar of Dementia twice by sacrificing Phyrexian Delver and Gray Merchant.
2. When Phyrexian Delver and Gray Merchant die, Prowling Geistcatcher's first ability triggers twice, exiling Phyrexian Delver and Gray Merchant from your graveyard.
3. Activate Altar of Dementia by sacrificing Prowling Geistcatcher.
4. When Prowling Geistcatcher dies, its second ability triggers, returning Phyrexian Delver and Gray Merchant from exile to the battlefield.
5. When Phyrexian Delver and Gray Merchant enter the battlefield, they trigger.
6. Resolve the Gray Merchant trigger, causing each opponent to lose an amount of life equal to your devotion to black, and causing you to gain life equal to the life lost this way.
7. Resolve the Phyrexian Delver trigger, causing you to return Prowling Geistcatcher from your graveyard to the battlefield and lose 4 life.
8. Resolve the Altar of Dementia abilities from steps 1 and 3.
9. Repeat.