This supplement updates some material from the 3.X era, as well as a little bit of new content. Most of it is centered on the Hornsaw Forest and those who inhabit it, with a couple of exceptions.
Overall, I found the quality really good. I liked the small comments from in-game characters such as Zarra or Yugman, it adds a little bit of spice. The paragraph on self-harm and mental health at the table was a really nice touch too. However, I could not rate this a 5 star review as I wanted to because of what I perceived as a few flaws in the design (either balance issues, or confusion wording in some places)
Arachnid Apotheosis => this feature is really underwhelming for a 17th level feature. It seems more or less as powerful as a cantrip, but it requires a) to make a melee attack (while the subclass doesn't have anything that supports melee attacks) and b) make a saving throw, and then the good effects only comes at c) when the monster reaches 0 hit points. For comparison, the venomous barb abilty of the Pact of the Serpent is more powerful...
EDIT : and now that I think about it, I would have preferred to see a Vengeance domain for Sethris rather than a Venom domain.
Tanil's Blessing => nothing really important, but there is a "per day" that slipped under the radar, and that is not really 5e-like (NB: I really like this subclass)
Ophidian Spirit => now that feature is confusing. It is not clear whether you have the benefit of all 3 abilities, or if you need to choose one. In addition, the duration of 1 hour/warlock level is kinda weird. Can you cumulate multiple use? Like use it once to get +3 to AC, then do a short rest, then use it a second time to get another +3 to AC? (Insert here a minor comment about the "per short/long rest" wording). It seems like this feature was shorten so that the subclass would fit the page, and I think some clarification are required ;)
Spells => some spells are never mentionned elsewhere, and there is no spell list per class, so it is not clear how one can access these spells. Maybe the combined book will have a spell list?
Bite of the mamba => I am not fond of ability score loss in 5E, I believe that the original designers avoided to use this tool for a reason, and therefore that it is better to avoid it. It is my personal opinion ;)
Sethris Potency => increasing a DC by 1d4 + proficiency bonus seems really powerful, even for a 3rd-level spell. DC in 5E are less spread out than in 3.X so I am a little bit cautious about that one. An alternative (the one I was thinking about when I drafted a conversion for that spell) is that you replace the poison DC with your own spell save DC. This globally provides the same increase for low-level poisons, but doesn't increase too much poisons with very high DC.
Entrance the beast => it is not clear if the first sentence is an actual ability or not (my bet is not, but if it is, there is a ton of information that is missing.
Entrance the beast / Snake eyes => these two are missing information about how often these can be used.
NB: these are mostly details, and again, overall the book is great ;)