Scenario: I took control of my opponent’s Sleeping Bear using Mark of the Goddess. Per the card, the bear would be returned to him when the spell was discarded “at the end of the round”, unless he didn’t have battlefield slots and then it would be discarded.
At the end of the round, his battlefield was full, but he had two Brilliant Thorns, which are destroyed “at the end of the round”. Which should be discarded first? Did he have battlefield slots available or no?