Accidentally let the final boss get a copy of High Priest Thekal and play it. They were at ~250 armor and 1 HP.
First weirdness: Bringing the boss down to 200 total health (hp and armor) triggered the new hero power, but set his HP to 200 and kept the armor. So now he had 399 effective health. Oh well, guess it'll be a grind.
Second weirdness: Managed to get through the armor and get the boss down to ~150 HP before he dropped C'thun. I expect the score screen to show ~150 HP left on the boss, but instead it shows 1.
The bosses trigger to advance them to later phases evidently only fires in response to damage events. Thekal's health setter is not a damage event, so it probably overlooks the change. Not certain what logic leads to the exact numbers we see, though. Need more reports.
Those who go on to lose these encounters claim that the boss is still left with only 1 HP in the menu, supporting the apparent intent that this value reflects the lowest health the boss had at any point in game.
https://www.reddit.com/r/hearthstone/comments/d6rei5/so_i_was_playing_with_high_priest_thekal_and_he/
https://www.reddit.com/r/hearthstone/comments/d7j2mw/weird_final_boss_interaction_with_high_priest/
The bosses trigger to advance them to later phases evidently only fires in response to damage events. Thekal's health setter is not a damage event, so it probably overlooks the change. Not certain what logic leads to the exact numbers we see, though. Need more reports.
Those who go on to lose these encounters claim that the boss is still left with only 1 HP in the menu, supporting the apparent intent that this value reflects the lowest health the boss had at any point in game.