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