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, Omnath's first ability triggers, creating a 5/5 Elemental creature token.
Activate Ashnod's Altar by sacrificing the Elemental token, adding ({C} magic symbol) ({C} magic symbol) .
When the Elemental dies, Omnath's second ability triggers, dealing 3 damage to any target.