When Stangg dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Stangg from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Stangg enters the battlefield, it triggers, creating a 3/4 Stangg Twin creature token.
Activate Ashnod's Altar by sacrificing the Stangg Twin, adding ({C} magic symbol) ({C} magic symbol) .