Cast Rot Hulk by paying ({5} magic symbol) ({B} magic symbol) ({B} magic symbol) .
Rot Hulk enters the battlefield, triggering its ability.
Holding priority, activate Carrion Feeder by sacrificing Rot Hulk, putting a +1/+1 counter on Carrion Feeder.
Resolve Rot Hulk's trigger, returning Phyrexian Delver and Gray Merchant of Asphodel from your graveyard to the battlefield.
Gray Merchant of Asphodel and Phyrexian Delver enter the battlefield, triggering their abilities.
Holding priority, activate Carrion Feeder by sacrificing Gray Merchant of Asphodel and Phyrexian Delver, putting two +1/+1 counters on Carrion Feeder.
Resolve the Gray Merchant of Asphodel trigger, causing each opponent to lose 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, returning Rot Hulk from your graveyard to the battlefield and causing yourself to lose 7 life.