Callous Bloodmage dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Callous Bloodmage from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Callous Bloodmage enters the battlefield, creating a 1/1 Pest creature token.
Activate Ashnod's Altar by sacrificing the Pest, adding ({C} magic symbol) ({C} magic symbol) .
When the Pest dies, it triggers, causing you to gain one life.