This is so far my favourite Tier 4 adventure to DM (and I DM'ed most of them : DDAL00-01, DDAL00-02F, DDAL00-03, DDAL05-18, DDAL05-19, DDAL06-03, DDAL07-15, DDEP08-03, DDAL08-16, DDAL08-17, DDAL08-18). Like the other modules in the trilogy, I have ran this one twice (both times with bonus objectives) : once physically at the Festival Magic Montreal, and once online. This review contains spoilers.
On the roleplay side, it was great ! Roleplaying a distracted mass of flesh named Berlain who was hurting the players without even realizing it (until the players gave her something else to tear apart) was great, and so was roleplaying a heart-broken demilich who could only say "Berlain" in a hundred voice tones. But roleplaying the Entity -- so, so fun ! The "Evil Gentleman" trope is so rare to see, but so satisfying to incarnate. Both times, the players loved the "Tell me what's missing" rooms and really got into it in terms of descriptions.
On the exploration side, it was great ! The Nightwalker corridor really pushed the players to get clever about who gets to go first, and their satisfaction was great when they figured out the Crossing Lines puzzle. The Ice Pillar and Waltz rooms of Bonus Objective A were both really cool to experience for them, and the maze sure got them on their nerves -- especially when the bardadin touched a wall, failed the madness save and got the "attack the nearest creature" effect !! (lucky for him, the cleric had Greater Restoration handy). The moral issue of having to bash their employer's "house" to continue also made for some interesting debates.
On the combat part... I'm not sure, because I modified them. I removed the entrance fight with the Star Spawn, because in both cases, I had previously served my players with many star spawn and did not want to make the whole thing feel repetitive. I also transformed, as explained above, the Berlain fight into a social encounter where she would pleasantly speak to them while other parts of her body would be trying to tear the nearest player apart -- unsettling but interesting. The Hannelore fight is where I went full throttle. Here is how I designed it : Four Boneclaws right next to the entry portal in a half moon to threathen all players with their long arms; Next to them, two Bodaks to passively hurt the party; Flying across the room, the suggested Adult Blue Dracolich; In the center of the room, Hannelore the Death Knight, next to a Skull Lord, an Alhoon, and a canoloth. The Skull Lord has casted a barred Forcecage to enclose himself & the alhoon & the death knight & the canoloth, and has casted Globe of Invulnerability. Since the Skull Lord passively regenerates undead within 30ft (the radius of the room, of which he's in the center), and since Hannelore has a Lair action to heal a dead boneclaw to full health, this fight certainly was something else ! The players won, of course (by Dispelling the Globe then blasting the caged opponents), but the unique design made them think, as simply bashing the nearest enemy was not the best way to go about it. For those who read this review, feel free to borrow this design for a more challenging fight ! As for the final Entity fight, I upped it up with the dramatic effect that "IT" was spawning Nightwalkers in a regular fashion (from a thematic standpoint, releasing those who were in the corridor at the start of the module). The first group managed to talk the Entity into accepting to be escorted to the Abyss and torture the ever-replenishing source of demons over there for the rest of eternity. The second group dealt with it by the Cleric successfully rolling for a Divine Intervention (got a 3 on the percentile die !!), which caused a dramatic scene in which the Entity slowly died from the inside screaming for help -- a wonderful end for a wonderful module.
I think this will remain my favourite module to DM for quite some time... maybe I'll go DM it a third time soon !