Activate Ashnod's Altar by sacrificing another legendary creature you control, adding ({C} magic symbol) ({C} magic symbol) .
The creature dies, triggering Ratadrabik and Nim Deathmantle.
Resolve the Ratadrabik trigger, creating a nonlegendary token copy of the creature that is a 2/2 Zombie in addition to its other types.
Activate Ashnod's Altar by sacrificing the Zombie, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, returning the legendary creature from your graveyard to the battlefield, and attaching Nim Deathmantle to it.