When Woe Strider dies, Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Woe Strider from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Woe Strider enters the battlefield, it triggers, creating a 0/1 Goat creature token.
Activate Ashnod's Altar by sacrificing the Goat, adding ({C} magic symbol) ({C} magic symbol) .