Prowling Geistcatcher's first ability triggers twice, exiling
Phyrexian Delver and Gray Merchant from your graveyard.
Activate
Phyrexian Altar by sacrificing
Prowling Geistcatcher.
When
Prowling Geistcatcher dies, its second ability triggers, returning
Phyrexian Delver and Gray Merchant from exile to the battlefield.
When
Phyrexian Delver and Gray Merchant enter the battlefield, they trigger.
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.
Resolve the
Phyrexian Delver trigger, causing you to return
Prowling Geistcatcher from your graveyard to the battlefield and lose 4 life.