So I, of course, have questions, now that I've taken a second look and compared it to the o5e version...
You've changed Enslave from 3/day, DC 14, to practically at-will, because it's a Legendary, to DC 16. That's going to increase the size of the aboleth's retinue of thralls by a
lot. This isn't a bad thing, mind, just a thing I noticed.
I do wish you had made Thrall into a template, although it isn't shouldn't be too hard to reverse-engineer it. I like templates.
The Sea Change disease can only be reversed by a disease-removing
spell of 4th level or higher. Does this mean other effects--such as the adept's ability to remove disease, or a magic item of disease-B-gone--also don't work, because it's not a spell?
I can't tell if this is the entirety of the aboleth entry, but what you've shown us lacks the lair actions of 05e's aboleth. Are you doing away with the aboleth's lair actions, or is it on the next page?
(Speaking of which, considering each entry seems to have a lot more info in it, what with the Legends & Lore, Encounters, Signs, and Behaviors, do you have an estimated page count for your MM?)
I will say I'm not a huge fan of your treasure tables. Obviously, I can ignore them at substitute my own treasure, but it's a weird image, that every aboleth encounter will result in a
periapt of health. (<Old Lady Voice> "In my day, aboleths had Treasure Type F, with its 10% chance of 3k-18k silver pieces and so forth, and we liked it like that!" </OLV>)
Shame on me I guess.
