When Redcap Gutter-Dweller dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Redcap Gutter-Dweller from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Redcap Gutter-Dweller enters the battlefield, its first ability triggers, creating two 1/1 Rat creature tokens.
Activate Ashnod's Altar by sacrificing a Rat, adding ({C} magic symbol) ({C} magic symbol) .