Sethron dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return Sethron from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Sethron enters the battlefield, creating a 2/3 Minotaur creature token.
Activate Ashnod's Altar by sacrificing the Minotaur, adding ({C} magic symbol) ({C} magic symbol) .