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, each Scute Swarm you control triggers, creating that many 1/1 Insect creature tokens or token copies of Scute Swarm.
Activate Ashnod's Altar by sacrificing a creature token, adding ({C} magic symbol) ({C} magic symbol) .