Holding priority, cast Dualcaster Mage by paying its mana cost.
When Dualcaster Mage enters, it triggers, copying Return to Action, with the copy targeting Dualcaster Mage.
Resolve the copy of Return to Action.
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 clever loop turns Shilgengar, Sire of Famine into an engine that feeds itself by looping a wizard's spellcasting prowess. By utilizing Return to Action to give a creature a built-in resurrection trigger right as it dies, you set the stage for Dualcaster Mage to enter the battlefield with flash and copy that very same spell onto itself. Every time Shilgengar sacrifices the Mage to fuel its ability and create Blood tokens, the death effect from Return to Action immediately brings the Mage right back to repeat the entire process, generating limitless death triggers, sacrifices, and Blood tokens.