I used the weapon that steals enemy minions if they survive on the headless horseman before he goes into his second phase.
Tested and confirmed:
Playing the card soft-locks the client like this. Restarting allows one to finish the game as the Horseman:
You also advance to phase 2 upon taking lethal damage:
Possible explanation for why this happens: The Kodo Hide Whip isn't written to check that you attacked a minion. Instead it checks that the thing you attacked isn't the enemy hero. When the Horseman advances to phase 2, his hero is sent to the setaside zone (with full health) and the Headless variant replaces him. This causes the hero you attacked to no longer be "the enemy hero", meeting the condition of the Whip.
https://www.reddit.com/r/hearthstone/comments/dg5sx7/small_chinese_indie_company/
Tested and confirmed:
Playing the card soft-locks the client like this. Restarting allows one to finish the game as the Horseman:
You also advance to phase 2 upon taking lethal damage:
Possible explanation for why this happens: The Kodo Hide Whip isn't written to check that you attacked a minion. Instead it checks that the thing you attacked isn't the enemy hero. When the Horseman advances to phase 2, his hero is sent to the setaside zone (with full health) and the Headless variant replaces him. This causes the hero you attacked to no longer be "the enemy hero", meeting the condition of the Whip.