I assume its a bug with how his phase two clears debuffs, and how it eats Maxium hp for 15 seconds. And when it happened the DOT damage happened instantly, while also instead of being given back the max hp that was taken it instead damaged him, so it dealt around 2.1k, and thats a little under half of phase two Malikeths hp, and so then he gets hit with three blade of death arts, which each deal 1.06k damage and ontop of 1k health of his being stolen.
