Check the latest information for Set 4: Fates!
TFT 10.19 patch data has been applied.
1 When The Boss first drops below 40% Health, he leaves combat to start doing sit-ups. Each sit-up restores 15% of his maximum Health and gives him 40% Attack and Movement Speed. If he reaches full Health he returns to combat Pumped Up: all of his attacks and spells deal true damage. If all of his allies die, he will immediately return to combat.