Haakon, Stromgald Scourge, Morophon, the Boundless, Nameless Inversion, and Rowan, Scion of War

Prerequisites

• Haakon, Stromgald Scourge on the battlefield
• Morophon, the Boundless on the battlefield
• Nameless Inversion in hand or in the graveyard
• Rowan, Scion of War on the battlefield

Steps

1. Activate Rowan by tapping it, causing black and red spells you cast to cost less for each life you've lost this turn
2. Cast Nameless Inversion from your graveyard, as it is a Knight card, by paying due to Rowan and Morophon, giving a creature +3/-3 until end of turn
3. If the creature has toughness 0 or less, the creature dies as a state-based action
4. Repeat from step 2 as desired

Results

• Reduce the toughness of creatures opponents control to 0
• Reduce the toughness of each creature opponents control to 0 upon entering