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, Seed the Land triggers, creating a creature token.
Activate Ashnod's Altar by sacrificing the creature token, adding ({C} magic symbol) ({C} magic symbol) .