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 sacrifice triggers.
Infinite death triggers.
Infinite colorless mana.
Infinite magecraft triggers.
Reduce any number of target creatures' toughness to zero.