Activate Ashnod's Altar by sacrificing Magus of the Bridge, adding ({C} magic symbol) ({C} magic symbol) .
When Magus of the Bridge dies, it and Nim Deathmantle triggers.
Resolve the Magus of the Bridge trigger, creating a 2/2 Zombie creature token.
Activate Ashnod's Altar by sacrificing the Zombie, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Magus of the Bridge from your graveyard to the battlefield, and attach Nim Deathmantle to it.