Sprouting Thrinax dies, triggering itself and Nim Deathmantle.
Resolve the Sprouting Thrinax trigger, creating three 1/1 Saproling creature tokens.
Activate Phyrexian Altar three times by sacrificing the Saprolings, adding ({3} magic symbol) .
Resolve the Nim Deathmantle trigger from step 2, causing you to pay ({4} magic symbol) to return Sprouting Thrinax from your graveyard to the battlefield, and attach Nim Deathmantle to it.