Thopter Mechanic dies, triggering itself and Nim Deathmantle.
Resolve the Thopter Mechanic trigger, creating a 1/1 Thopter artifact creature token.
Activate Ashnod's Altar by sacrificing the Thopter, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Thopter Mechanic from your graveyard to the battlefield, and attach Nim Deathmantle to it.