Adding an encounter into Tamoachan.

[Finback]

My group of 4 level 8s are exploring Chult at the moment in ToA, but one of the players who joined more recently created a backstory for his gith ranger, that he is hunting a particular illithid. So, I decided that I'd see a dungeon at some point and put that foe in there - decided on an ulitharid as appropriate for the group.
I also figured I'd use Tamoachan from Yawning Portal, since it's pretty much perfect to seed into Chult - but my question is - where would other DMs put this encounter, within the dungeon? Obviously, one doesn't put it right inside the front door, but I want to seed it somewhere where it will feel apt. I imagine the ulitharid is exploring ruins for portals for staging infiltration of Faerun, but also lookng for ancient artifacts.

Any recommendations?

[Spectrulus]

The original module had an ancient vampire trapped in his tomb, guarded by aggressive local animals, zombies in the inner parts, as well as animated statutes.

Sounds decent to me still. If you wanted to take it a different flavour, you could try Yuanti who can't detect the intruder, lizard folk too scared to fend off the mind flayer, an aboleth with ancient secrets the illithid desired and of course accompanying Chuul, deepspawn, etc.