When Dualcaster Mage enters the battlefield, it triggers, copying Black Sun's Twilight, optionally choosing to target a creature.
Holding priority, activate Phyrexian Altar by sacrificing Dualcaster Mage, adding one mana of any color.
Resolve the copy of Black Sun's Twilight, giving up to one creature -5/-5 until end of turn and returning Dualcaster Mage from your graveyard to the battlefield.
Repeat from step 3.
Results
Infinite LTB.
Infinite ETB.
Infinite colored mana.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite magecraft triggers.
Reduce any number of target creatures' toughness to zero.