Darth Nihilus annihilate with lead

From using it in the event, I think he does get the max health AND health, but only enough to keep him at the same percentage. Just like Ploosh said. His health bar won't look any different, but by the end he's a TANK. I don't have a good Sith/FO squad, so on tier 3 encounter 8, it would often get down to just him being stunned or otherwise not taking a turn, and the five Jedi would all take potshots at him. And it would seriously take them each 5 turns to wear him down. Maybe more. I didn't really bother to count. That's at least 25 hits and maybe in excess of 30. So the max health add was definitely working.