When Dryad Arbor dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Dryad Arbor from your graveyard to the battlefield, and attaching Nim Deathmantle to it.
When Dryad Arbor enters the battlefield, Springheart Nantuko triggers, choosing not to pay and creating a 1/1 Insect creature token.
Activate Ashnod's Altar by sacrificing the creature token, adding ({C} magic symbol) ({C} magic symbol) .