Brood Monitor dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Brood Monitor from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Brood Monitor enters the battlefield, creating three 1/1 Eldrazi Scion creature tokens.
Activate Ashnod's Altar by sacrificing an Eldrazi Scion, adding ({C} magic symbol) ({C} magic symbol) .