I wanted to like this adventure I really did, the dungeon crawl section in the middle is excellent and the only reason I didn't give this one star. The void is an excellent feature/trap, I had the party wanting to go through without the amulets to see their fears, which was nearly their undoing.
But then there's the rest of the adventure.
The story has potential but there isn't enough of showing it to the characters, it's all in the background which normally isn't read by players as they often contain spoilers. Klevin doesn't come across as the non-native who has assimilated into chultan culture that he was in City on the Edge. Instead he's a semi-incompetant cultural appropriator who rolls his eyes when non-chultans don't speak chultan.
Arbitary travel times are frustraing to everyone and anyone in modules, it shouldn't take a month to get to somewhere during them, unless your intention is to ensure the ceric gets at least two divine interventions in. Give them an air ship or some other mode of travel, we're not in tier 2 anymore.
The are far too many typos within the box text, the one thing DMs are expected to read aloud. The bare minimum of editing should ensure that these are eradicated.
The section where you meet the yuan-ti in the village is interesting, but falls down as it appears to be half finished. Why would the yuan-ti be ok with adventurers entering their sacred temple? Where are all the guards? If only Nightmare Speaker Nozhi decides who can enter, why do the party have to enter and clear the dungeon to ask for his permission?
The dungeon craw itself is great, if a little lacking in variety, a character who is immune to poison is going to ahve a very easy time with all but two of the traps. I would also reccomend some sort of map for the skull room as it isn't clear where the non-tabaxi skulls should be. The meeting with Satzhesu is an interesting departure to the usual "Meet the npc and get an information dump" however the limitations in what the DM is allowed to do for pantomime is ridiculous. No one is can do pantomie in a dignified manner, that's the point. Also the module expects the party to ask questions that they have no way of knowing they should ask because of the lack of story within the adventure.
Lastly, the final fight. Really the only fight unless you drop the optional encounter on the party. No it's possible to have modules only have one fight as long as they either sap resources other than HP (at tier 3 there's more than enough potions to ensure that the party will be at full in every encounter) or make the fight challenging enough that it doesn't matter that the party has all their resources.
Two CR 7 monsters is not challenging to an APL 13 party, not even close.
Yes one of them hs legendary actions, but only one is useful and you can only use it once per turn. Regaining spell slots means nothing when the party will kill them in one round.
Meanwile we have a bag of hit points, who while having the multiattack feature allowing three melee attacks, can use his only two melee attacks once.
So some suggestions to make this fight interesting:
Ensure that you incorporate the yuan-ti racial traits into the statblock of Nozhi, he should be immune to poison damage and have magical resistance. Also give him some legendary resistances else the party will feeblemmind him or worse in the opening round. Then change his Spell DC to something more threatening than a 15, a 17 or 18 is far more tier 3 appropriate. His spell list is actually very good so that can stay, though it might be worht throwing counterspell in there for good measure.
Ihl should also be very dangerous. How about changing them to an Anathema? Now we have a true threat to the party on the field and it's far more clear why a temple was built around their grave. If you really want to up the danger, how about some lair actions? Something that causes the void trap to hapen again would be incredible.
Lastly we need some mooks in there to soak up the fighters. Yuan-ti broodguard work great for this and you can adjust the numbers of them to the party strength.
This module has so much potential and I was really disappointed in what a mess it ended up being in the final encounter. If this module does get updated like 7-11 did I will definitely consider running it again.