When Eternal Scourge dies, Kinzu of the Bleak Coven and Dross Harvester's second ability trigger.
Resolve the Dross Harvester trigger, causing you to gain two life.
Resolve the Kinzu of the Bleak Coven trigger, causing you to pay two life and exile Eternal Scourge from your graveyard to create a token copy of it that is 1/1.
Activate Ashnod's Altar by sacrificing the Eternal Scourge token, adding ({C} magic symbol) ({C} magic symbol) .
When the Eternal Scourge token dies, Dross Harvester's second ability triggers, causing you to gain two life.
Cast Eternal Scourge from exile by paying ({3} magic symbol) .
Repeat.
Once you have infinite life and colorless mana, you may forego sacrificing the Eternal Scourge tokens.