As I just finished this boss fight for the first time the other day, I will recount what I did (I believe there will be useful info for those struggling with the fight). For those wondering, I found it not that difficult (Balanced Difficulty) and pretty sure I could handle it on harder diffculty knowing what to expect as I had no prior knowledge of the boss, and also see a method to handle the Legendary Action in Honour Mode that I will mention later. I can see why people found it anti-climactic or underwhelming, but have suspicions on why many found it hell while I didn't, which I will again also mention later.
So first, my party set up. The party was level 9 going in. My Tav is a Githyanki Life Domain Cleric, with all her gear centered on buffing her healing and granting free bless, and 21 AC. I brought Shadowheart, who I had reclassed as a Death Domain cleric, with I believe 22 AC, the ring that gives mental fatigue, and the Infernal Rapier which I had just got. Speaking of, I also brought Wyll (I am not sure if he needed to be there to get the Infernal Rapier, but did not wish to risk it) and despite what you may be thinking, he is not a warlock. For the purposes of this fight, he is a Way of the Shadow Monk, because I really wanted infinite teleports, with the Rapier of Screams. He is the weakest of the party with 18 AC. And lastly, Minthara, who I made a Oath of the Crown Paladin and Hexblade Warlock, with the Radiant Orb set and Blood of Lathander and a headset that gave her arcane synergy and 23 AC, by far the most powerful of those I brought.
So, my party goes in and starts fighting Thorm. I start sending Wyll to go handle the four Intellect Devourers and free the Nightsong. Minthara has the darkbow and so applied haste to herself. Shadowheart summons the Cambion. I forget what my Tav was doing, ngl, bear with me if I can't recount everything, I remember the notable things.
What happened is that Thorm stunned multiple of my characters, including Minthara and making her lose her haste immediately. Wyll, who has that ice armor temporary hit points spell applied, is dealing with the Mind flayers pretty well. Those few Necromite's in the corner were certainly a pain as they kept targeting my Tav, but thankfully a lot of them were saved (As in my character kept dodging).
I initially tried to use the Cambion to attack thorm but quickly realized that, one, he was invulnerable and I was wasting my time, and two, the Cambion had piss poor accuracy against him and it simply wasn't worth it. I instead used the Cambion to help with the Mindflayer, who honestly I ended up killing so quickly that he was barely noticeable. I had my Tav use spirit guardians when Thorm tried summoning more necromites, which wasn't the best move since I couldn't damage the egg things but did counter them effectively enough. (There was also a time where I used beacon of hope and then a widespread heal, but I forget where in the order this happened)
I then tried to have Shadowheart use Cloudkill on the necromites on the platform. This was a terrible idea as I forgot that they are immune to poison. I ended up moving the cloud to try and damage Thorm, and no idea if that really did anything of use. Wyll has killed the intellect devourers and freed the Nightsong by this point, so I have Minthara start damaging him heavily. The Nightsong was... pretty useless. She got killed by Thorm in one set of attacks. Which some of you may be thinking is fine, she'll be back. Just you wait.
So, Minthara then smites Thorm and he is near dead. And so comes the Avatar of Death popping up, to my surprise as I've only seen a clip of this thing existing and was not expecting him. I look at his stats and see his abilities, and quickly made one of the two decisions I think made this fight much easier for me. I had Shadowheart put silence on him.
I have not seen anywhere saying that Silence affects this boss in any notable way. I could possibly be wrong. But I see no other reason for why throughout the entire fight, he does not use an ability that normally causes havoc. I saw the effect on him mentioning that he can sacrifice necromites to use a move that basically means insta death for most characters, and he summons the damn things, and he did not do so once throughout the whole fight. I even got an achievement for that. I suspect this is because I put the Silence over him, but if someone knows that this is not the case, feel free to correct me, but I do believe it is likely.
I at this point send my Tav with spirit guardians and try to have her use her Triple githyanki given jump to get to the platform the Necromites were on to try and kill them all. A more effective thing to do would have been to Mistystep there, but I did not think of that and am generally conservative on resources. And at this point, the boss nearly one hit kills Minthara, a character I so far have never had go below half health (She was at ~60 health when this happened of her ~80)
I wisely have Minthara move out of the bosses' Bonechill zone so she can start using some heals on herself. The Boss at this point is still targeting her and making her frightened. Also at this time, I am using the Cambion to target the not yet spawned Necromites, because it's accuracy is again pointless against the boss. I'm also trying to maneuver Wyll into the other corner to handle the Necromites on the other side.
The Avatar of Death proceeds to inhale and drag my tav character to him, and I have my Tav first try to use a heal spell, to which I then remember his bone chill aura, and then realize that, the Nightsong has been and will be effectively removed for the whole fight because she died right where that aura prevents her from getting back up or helped. Again, the Nightsong is useless and literally just is not a part of the second phase. However, this did make me realize just how bad this was, and I have my Tav do a column of fire on the boss to do a good chunk before using Sanctuary to have her leave and help Minthara heal.
And at this point comes the second move that I suspect helped make this fight easy. Summons. The Boss was still targeting Minthara, and she is by far my most powerful character, so I need to get her healed and protected from the frightened effect. To this end, I have Shadowheart summon spiritual weapon. This helped take the aggro of the boss away from Minthara. I summon spiritual weapon twice, and I don't think even one got a chance to attack, but that's fine, they served their purpose. I also have my Tav summon a Guardian of Faith, which ended up being a free 60 damage (None of the boss's attacks hit it). I also summoned one skeleton with Shadoweart which did a bit of damage.
This is something I think would have helped even in the honor mode run, as I think the summons might be able to take his Legendary Action, which is a counterattack once per round. Something to consider.
And after this, the fight was basically smooth sailing. Wyll was handling the necromites on the left at this point, Cambion those on the right, Shadowheart was keeping up the silence and using the Toll the Bell cantrip to apply more mental fatigue, and Minthara, I kept away because I did not want her getting caught in the no healing aura, and she was able to do plenty of damage at a distance with the Warlock cantrip while applying lots of Radiant Orb to make the boss less accurate. I was able to have my Tav skip turns at the end because there was nobody to heal or do an action. (Minthara at this point put a protection from good/evil spell on herself to prevent being frightened if hit)
So main recommendations for this fight is use a silence bubble as fast as you can and then start bringing out summons to keep your party safe and use a damage dealer from afar. Again, this was my first time battling the boss, with zero prior preparation or knowledge. The spells they had were the spells I normally always keep on hand because I know their utility may come in use unexpectedly. Nor was this a fully optimized team, as Wyll is the weakest character I have as I am experimenting with him on an effective Shadow Monk build and have not figured it out yet, I brought him solely for his quest and the Infernal Rapier, which I was glad to have for that fight.
I do not believe this fight was all that difficult, though I do acknowledge that had the boss actually managed to get Minthara that first time, I may have been screwed. But I also suspect I would have managed, based on many of the decisions I made that I believe made the fight successful were very much reactive to the information I had, and I already had a thought on how to bring a character away from the no healing aura to bring them back, so not an impossible hurdle to come back from. I also had plenty of resources left over I could have used, like Blood of Lathander's sunbeam, or any divinity charges. I also didn't use any elixers, because I'm a stingy hoarder.
The fight was fun. Hope this was helpful.
