In my weekly home game, we're about to start the final chapter of Tomb of Annihilation (the erstwhile tomb itself). We just finished "Fane of the Night Serpent", the third chapter. Tomb of Annihilation is the best written campaign adventure for 5E, particularly if you like older adventuring styles. The campaign features 4 general story arcs - free form exploration across a sprawling jungle hex crawl; discovery and exploration of a lost ruined city sunk in a jungle chasm; infiltrating a subterranean fortress of Yuan-Ti in the ruined city (the Fane of the Night Serpent); concluding by finding and entering the Tomb of Annihilation itself. If you're a long time D&D player familiar with TSR's history, I'll point out the hex crawl sections of Chult are very reminiscent of X1 The Isle of Dread from the 1980's Expert Set. The ruined city and Yuan-Ti fortress are nods to I1 Dwellers of the Forbidden City. The Tomb of Annihilation itself is a riff on S1 Tomb of Horrors. It's several fantastic old school homages in one big campaign.
As the players explored the ruined city, it became apparent they needed to collect nine stone cubes from various trap-filled shrines in order to enter the Tomb. They were harassed by ambushers and spies of the Yuan-Ti while exploring the city, culminating with the final shrine, where they learned the cube is already gone, and a taunting message from their Yuan-Ti adversaries dared them to come to the Yuan-Ti lair, the Fane of the Night Serpent.
There's no right answer on how to prosecute a conflict against the Fane. There are ways to infiltrate it stealthily, and characters in the level 7-8 range have various magic resources to come up with an appropriate plan - disguises, invisibility, trickery, scouting for back doors, you get the idea. Not my guys - they opted for your basic "full frontal assault". It's 5E after all, and they're practically comic book heroes. (My old-schooler in the group frequently gripes his dwarf cleric is basically a "Villains & Vigilantes" character with super powers).
The entrance to the Fane is a 20' tunnel beneath a ruined palace, ending in colossal metal doors on tracks. They used Stone Shape to warp the walls around the main gates and create an entrance point for characters to squeeze one at a time around the edge of the door, and treated it like their own "once more into the breech" moment.
It was quite challenging. There are pits with snakes right on the other side of the gate the players didn't account for, and not everyone was ready to jump the pits after squeezing around the gate. There was also a handful of reptilian bruisers on guard duty (Yuan-Ti Brood Guards) and a boss, and a few of the characters that made the jump actually got shoved immediately backwards into the pits as a tactic by the brood guards. With all of the "Fey Step" spells available to the group, the pits and the door breach didn't delay them too long, but they quickly found themselves pinned down and trying to hold their "beach-head" on the far side of the gates.
In another nod to "old school ways" the Fane has a temple roster that lists how all of the inhabitants respond when the alarm sounds. It's very reminiscent of the defense roster in WG4 Forgotten Temple of Tharizdun, a Greyhawk module where the players encounter an all-out defense when they breach the Forgotten Temple. Those 1E adventures demanded that players be competent war-gamers and tacticians*. One of the bosses even telegraphed that an alarm would be sounded, literally commanding an underling, "You - go alert the complex!", in Draconic. For whatever reason, the players didn't prioritize taking out the runner, and a round or two later a gong echoed throughout the underground vaults of the Yuan-Ti. The fight quickly bogged down into a Tharizdun-like siege as monsters flooded in from different parts of the dungeon. (Note - the hardback book does not provide how many rounds it takes each group of monsters to respond, so that's a bit of work the referee needs to do to stagger the incoming forces - I used a blend of movement rates, distance from the entrance, and randomization that took into account what they were doing when the alarm was sounded).
My players were mentally unprepared to retreat. When you've known only victory, you cannot tell you're outmatched until it's too late, like frogs in heating water. If this were 1E their innate paranoia and fragility would have convinced them to bail a long time ago instead of assuming their plot armor would carry them through. Instead, the Yuan-Ti "Nightmare Speaker", Fenthaza, the high priestess, joined the fight (with her controlled Air Elemental) as well as the Yuan-Ti champion, a brutal warrior named Sekelok. By this point the players had defeated 15+ brood guards, probably 6+ Yuan Ti Malisons, a few basiliks, a rampaging triceratops... charred bodies were piled in the halls, smoke stung their eyes. But resources were dwindling, mistakes were made, and the dwarf cleric went down beneath Sekelok's pounding two-handed sword. Take out the healer and the 5E elevator stops working. The players noted the Yuan-Ti were avoiding killing blows. "Keep them alive", Sekelok was roaring to the others, "their blood will feed Dendar the Night Serpent on the sacrificial stone".
I was able to savor the utter disbelief across my player's faces as more Yuan-Ti descended on the entrance siege and one by one, the player characters fell into unconsciousness beneath the terrible beating. It was better than a TPK since we don't even have to restart the campaign! It's okay readers, take a moment and gloat along with me - you can do it. The characters awoke stripped, manacled, and bruised, in darkened fetid pits looking up at dim grates high above their heads - prisoners of the Yuan-Ti. I guess that's one way to get into a fortress.
When we regrouped the following week, the players had to deal wtih being in separate pits, with other prisoners of the Yuan-Ti (not altogether stable or sane), and try to piece together what they could learn about the Yuan-Ti's plans. They'd be given the chance to undergo the Yuan-Ti transformation (if human) and join the monsters as reinforcements, or sacrificed (if non-human) or made into a mindless slave. This became a fairly roleplay heavy session. The Yuan-Ti spymaster eventually came to interrogate them, and had one of the mouthy characters, Woodson, dragged before the boss - the ancient undead warlord Ras Nisi. Stories of Ras Nisi had been circulating in the campaign lore since the beginning, so it was a momentous meeting. Furthermore, Ras Nisi was rotting away, like a mummy! Woodson figured out he had the Death Curse, and was dying just like their patron. If you're unfamiliar - the Death Curse that initiates the campaign is unraveling Raise Dead and Resurrection magics, like a form of magical leprosy, and blocking all future Raise Dead or Resurrection magic across the world.
The players were able to convince Ras Nisi their mission to defeat the Death Curse would fix him, too... They knew he had a harem of Yuan-Ti lovelies, "clearly your concubines find your current state repulsive, when you recover you'd be able to know a gentle touch once more..." that kind of stuff to appeal to his vanity and self-preservation. Great role-playing, and they were able to cut a deal with the Yuan-Ti boss to take out his rival (the awful Nightmare Speaker, Fenthaza) and continue their mission against the Death Curse. They also learned a critical piece of in-game lore - Ras Nisi's ally is an ancient lich, Acererak, and the Tomb of Annihilation is one of his legendary death trap dungeons! Now Ras Nisi believes Acererak betrayed him because of the Death Curse. The players already knew Acererak was involved somehow - there's literally a "Green Devil Face" on the cover of the book - but now they could port in all their Acererak lore above board at the table.
Ras Nisi's minions arranged a plausible pre-dawn jail-break along with a rough map to Fenthaza's lair, and the players developed a plan to sneak in and kill her. (Now they finally remembered to use the Invisibility spells, their Silence 15' Radius, their Helm of Telepathy, etc etc). They took down the Nightmare Speaker and her various bodyguards and warlock attendants in short order, earned back all of their stone cubes (including the precious 9th cube they needed from the Yuan-Ti) and escaped into the grey morning mists of the city, free once more.
Collectively, the players remarked these were the best two game sessions in recent memory - plenty of emotional highs and lows, some hubris was cured, but in the end smart play, good role-playing, and effective tactics got them back on top. That concluded Sessions 26 and 27 for our Chult game. Next up - they Tomb of Annihilation itself!
*I've found that G-1-2-3 can also devolve into dungeon sieges, especially the entrance to the Fire Giant dungeon, and some of the Drow and Kuo-Toa locales in the D-series as well. Very nice that Tomb of Annihilation contemplated a dungeon siege response.