You control at least one additional nontoken Dragon.
({2} magic symbol) available.
Steps
Activate Ashnod's Altar by sacrificing another nontoken Dragon, adding ({C} magic symbol) ({C} magic symbol) .
The Dragon dies, triggering Nim Deathmantle, causing you to pay ({4} magic symbol) to return the Dragon from your graveyard to the battlefield, and attach Nim Deathmantle to it.
The Dragon enters the battlefield, triggering Lathliss, creating a 5/5 Dragon creature token.
Activate Ashnod's Altar by sacrificing the Dragon token, adding ({C} magic symbol) ({C} magic symbol) .