Dualcaster Mage + Black Sun's Twilight + Phantom Train

Dualcaster Mage

Black Sun's Twilight

Phantom Train

How it works

1. Cast Black Sun's Twilight by paying 5
2. Holding priority, cast Dualcaster Mage by paying its mana cost.
3. When Dualcaster Mage enters, it triggers, targeting Black Sun's Twilight.
4. Holding priority, activate Phantom Train by sacrificing Dualcaster Mage.
5. Resolve the Dualcaster Mage trigger, copying Black Sun's Twilight
6. Resolve the Black Sun's Twilight copy, returning Dualcaster Mage from your graveyard to the battlefield.
7. Repeat from step 3.

This clever loop sets up an infinite recursion by leveraging Dualcaster Mage's spell-copying entry trigger to continually recast and copy Black Sun's Twilight with an $X$ value of 5 or more. While Black Sun's Twilight provides the heavy lifting by reanimating our wizard directly from the graveyard, Phantom Train acts as the crucial sacrifice outlet that feeds the engine and keeps the sequence going. By continually sacrificing Dualcaster Mage to the Train, you not only fuel endless sacrifice and death triggers but also pump up the Phantom Train with a steady stream of +1/+1 counters.