Excellent attempt at "free"-ing up the original game engine, for OGL usage. The layout is clear and easy to follow. The rules are more streamlined, compared to other alternatives.
I especially like that they provide guidelines for the Game Moderator to construct their own tomes, spells, monsters based on simple guidelines. The creators also present conversions of existing monsters, spells, tomes on their website https://cthulhueternal.com/open-mythos/

Absolutely adore this unique setting book. It's exactly what I've been thinking about exploring in one of my campaigns, and this is a treasure trove of ideas. It's also very usable, with great layout and concise writing. The system for advancing the development of the "apocalypse threats" is also great