Hearthstone - Power Overwhelming and Shadow Madness

I was just playing what I can only assume was priest control deck against my pseudo zoo lock.

I had a couple of Jugalos out, a Blood Imp, a Voidwalker and something else.

He played shadow madness against one of my Jugalos and then gave it power overwhelming. so it was a 7/6. He then smashed my Voidwalker in the face with it, dropping it down to a 7/5.

He ended turn expecting it to die, except it returned to my deck. Presumably because the mechanic that would return it to my deck at the end of the turn was played before the power overwhelming. I guess he then expected it to die.

Except, it must have done this as the turn was ending and landed in my deck after his turn had ended, because once in my deck it didn't die!

He then emoted "Oops": I said "Thanks!"

I don't know if it would have died at the end of my next turn as I then smashed him in the face for in excess of 15 damage on what I think was turn 5!