They have a pretty hefty DoT that can't be mitigated, and they are defiled so much that they are effectively unhealable. The DDs reported having trouble seeing it at times.

Pre-split, the biggest pain point is the Sundered Soul. We'll give this last boss some more tries later in the week. We reached that phase a few times, and then decided that we wanted to have some time for the other dungeon, so we turned off HM. We haven't gotten past the first split phase at 75%. they were comparable to triple-HM dungeons before that), this boss was a bit of a shock. After the relatively easy first two bosses (by Ascending Tide standards, at least. The hardest part about the fight was finding the right camera angle to be able to actually target and hit the boss. The main problem with boss 2 is that the boss's hitbox becomes very weird when he reaches the larger sizes. So, completely ignoring that mechanic and just killing the boss while sidestepping the red seemed like something that should work. There's an achievement for allowing the boss to grow to its maximum size, and there's an achievement for not killing the skeletons that empower it. This boiled down to a tank-and-spank-while-sidestepping-red. The beams were not helpful on any of the subsequent bosses and might as well not exist. considering the amount of effort required to get it. The special effect unlocked by this secret boss seems. Let's just say that I'm glad that there only needs to be one person who's good at doing spy-movie-laser-beam gymnastics, because I'm not that person. The boss jumps around a lot (much of the consternation of DDs who these new-fangled long, slow-ticking ground DoTs), and the Battered DoT was rather painful, but on the whole the fight seemed okay. Which I think is fine: there should be a progression of difficulty, and this is the first boss, after all. This boss is considerably easier than the first boss HMs for the Ascending Tide dungeons and is comparable to the first boss HM of Stone Garden. If there is meant to be a solution other than 'keep trying each option until it works', it's not something that we were able to find. We searched around for clues for the solution to the puzzle, but were not able to find any. We solved the puzzle by brute-force trial-and-error. The aesthetics, the underwater walking, etc., are by far the best we've seen. We ran this dungeon first because Finn's preview on ESO Live suggested that this is the 'cool' dungeon. Haven't had time to really process them these are just some initial thoughts.

Eldest souls secret boss