Holding priority, cast Dualcaster Mage by paying its mana cost.
When Dualcaster Mage enters, it triggers, copying Presumed Dead, with the copy targeting Dualcaster Mage.
Resolve the copy of Presumed Dead.
Holding priority, activate Shilgengar by sacrificing Dualcaster Mage.
When Dualcaster Mage dies, it triggers, returning it from your graveyard to the battlefield.
Repeat from step 3.
Resolve all Shilgengar abilities from step 5.
This intricate loop relies on Presumed Dead targeting your flash-enabled Dualcaster Mage right as it enters the battlefield to copy the spell itself. With the self-recurring safety net established, Shilgengar, Sire of Famine acts as the engine's heart, letting you sacrifice the Mage to fuel its ability and immediately trigger its return from the graveyard thanks to the lingering effect. Every iteration of this cycle pumps out infinite Blood tokens and death triggers, letting you overwhelm the board and bury your opponents in endless sacrifices.