Dualcaster Mage's ETB ability triggers, targeting Living Death.
Activate Spawning Pit once for each creature you control by sacrificing that creature.
Resolve all Spawning Pit abilities, putting a charge counter on Spawning Pit 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.