Dualcaster Mage's ETB ability triggers, targeting Living Death.
Activate Phyrexian Altar once for each creature you control by sacrificing that creature, adding one mana of any color each time.
Resolve Dualcaster Mage's ability, copying Living Death.
Resolve the Living Death copy, causing each player to exile all creature cards from their graveyard, sacrifice all creatures they control, then put the exiled cards onto the battlefield.