Great little product especially for the price. 13 thematically-linked aberrations, twelve of them in the challenge 3-7 range which is great for ramping into Mind Flayer or Slaad encounters. Five of them are updates of the Foulspawn published by WotC for 4th edition, and you can see some callbacks to the 4E mechanics like Bloodied. The other monsters are variations on those originals, or creatures that fit the theme and support the ecology you can build around them.

This has the closest I've seen to a ready-made jump scare in a D&D product: a small, pathetic creature described "as docile and subservient" hiding in the corner, that can suddenly lash out for 55 damage at range, teleport away, and hide. The party's barbarian is on his back making death saves before anyone knows what's happening. That's great in context of monsters designed around a horror theme.

But in the hands of a rookie, that same monster only being Challenge 3 is a bit strong and likely to lead to a lot of unintended character death. I disagree with some of the mechanical balance here, it's easy enough to adapt but I wouldn't recommend using it as-is to a new DM. That's the only reason I can find to remove a star.

There are some minor typos and copy-paste errors (the Surgeon's ability being reused on other monsters is fine, but make sure to change the word "Surgeon" in the ability text when you copy it). There are a couple of places where clarity would be useful (when does the vulnerabilty from the Warpcaller's pipes end? One round? Short rest? Permanently? Until true love's first kiss?). But for an independent author with no editor or development staff, you can't really fault that.

My advice: buy it, but use the Challenge listings as very light suggestions only, same as with the official monsters (Intellect Devourer killing a stunned creature outright, no saves, at Challenge 2? Really, WotC?). Remember to look at what the monsters actually do before you thrown them against your party, be ready to adjust some minor things on the fly, and you'll be fine.