When Rendmaw dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Rendmaw from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Rendmaw enters the battlefield, it triggers, causing each player to create a tapped 2/2 Bird creature token.
Activate Ashnod's Altar by sacrificing the Bird, adding ({C} magic symbol) ({C} magic symbol) .