I used Harland Mindlock to control an enemy flank creature. But if Harland Mindlock leaves play, I do not control the controlled creature anymore. What happens to the controlled creature? Is it going back in opponent hand? Or maybe opponent discard pile? Or back to enemy battle line? Help appreciated here thank you.

https://decksofkeyforge.com/cards/harland-mindlock

When Harland Mindlock leaves play, the opponent regains control of the card, so it returns to their battleline, on a flank of the active player's choice.

Battle line in this case, and the active player decides which flank it goes to.

thank you!