Oh dear, he'd let slip more than he'd meant to.

"Just a little accident," he said making it sound as harmless as possible. "Played a prank on Czernobog that got out of hand and ... would you believe he accidentally cursed me? He wasn't even sure what he'd done so he couldn't undo it and I had to go to the humans for help."

"It would have been a great prank, if I'd only managed to knock Czernobog out properly, though."