He probably used "Letho: Kingslayer" (transform into another card without changing power)My opponent just used 2x Damien De La Tour in one round... Is it even possible? It happened to me more times today with golden cards...wtf?! Is it a cheat?
There are some other cards that could do that but they're mostly RNG create cards.
Finally, he could have seiced yours, if you had one, too.