I was busy keeping track of Black Tentacles and Crushing Despair's effects so I forgot about the second save for creatures who critically fail against Phantasmal Calamity, which could have stopped the two standing PCs from reviving the two downed PCs and led to a complete TPK. Just one of them failing the save would have guaranteed a dead party member, and they only had about a 20% chance each to succeed. After that they would have had to roll a 19 or 20 to escape the illusion thanks to their debuffs, which they'd need to get on their first attempt or lose both of the downed party members.
And now one of my players has pointed out Black Tentacles has been upgraded to Slither in the remaster so they should have been even more killed. With an extra 2d6 damage plus persistent poison every round I could have absolutely tpk'd them. This is so fucking upsetting. They should have all died and had to make new characters and the paladin player forced to write the backstory on why the help he requested took so long to arrive 
i just wanna kill PCs and everyone said this AP would kill loads but I trained my players too well and now I missed my chance to kill them aaaaaaaAAAAAA 
Lessons learned
I will always cherish the time I had a "friendly" NPC hand one of the players a scroll to "get them out of a jam"
They thought it was a teleport scroll, but not the kind they thought it was
It was an Anywhere but Here scroll, and they ended up teleporking themselves to Akiton and fighting a band of Shobad raiders who shot them up good
Oh, that's mean. A long time ago I reminded my players they had collected a bottle of liquid from the alter of an evil god earlier in the dungeon when a boss fight was going extremely south, so they fed it to the unconscious fighter on the basis that whatever it did, it couldn't make things worse.
The Waters of Lamashtu corrupt any who drink it, and in extreme cases can permanently transform people into horrifying abominations. One poor roll later and the boss had a brand new minion.
Amazing, i love it!