Opponent has
Triumph of Cruelty in play. When none of us have a creature on the Battlefield, nothing happens during the upkeep.
When I play a 0/
X creature, on the next upkeep
Triumph of Cruelty forces me to discard a card, but the controller of
Triumph of Cruelty has no creatures. This seems unfair.
Reading the text, I would presume the controller of
Triumph of Cruelty must have a creature to "control the creature with the greatest power", so I'm guessing Forge compares 0 power on my board to 0 power on opponent's board, but forgets to check that we both have a creature.