Nim Deathmantle triggers, causing you to pay ({4} magic symbol) to return Myr Battlesphere from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Myr Battlesphere enters the battlefield, creating four 1/1 Myr creature tokens.
Activate Phyrexian Altar by sacrificing three Myr tokens, adding ({3} magic symbol) .