I am meaning to work out a reasonable way of creating a 'monster class' using the eidolon mechanics. I am not so sure about balance so I would welcome aid in creating a decently balanced 20 level class.
This may not fit all settings as I've made it for my own, but here's the basic blurb I'm using:
History is filled with tales of demons and monsters from other worlds. What may be more terrifying than the tales themselves is that almost every one of these beings was once human. Whether by a return from the dead gone wrong or a natural connection to the astral sea, these tragic individuals warp beyond recognition into supernatural things from nightmares. Some can be possessed of such power unknowingly, the transformation occurring at an unexpected point in life.
Termed 'astral avatars' by scholars of extraplanar lore, these transformed entities take a form subject both to their own subconscious and the unthinking and diffuse dreams of dead souls on the astral plane. They are avatars of imagined things, and in fact it is peoples' perception of the common demon that most often shapes them. This essential tie to the astral sea grants them abilities beyond mortal capability and they are all fearsome combatants.
I have a mock up document for the class that is playable, but I've put no work into fine tuning the balance yet. Most of it is cut and paste from the eidolon. As a note, I didn't like the term 'evolutions' so I called them 'aspects' for my game. This remains in the document here.
I've given sorcerer casting with ranger spell progression, and intend to give the astral avatar a cut down spell list, but haven't put any thought into what kind of spell list I want to aim for. At the moment it's the full sorc/wiz list up to 4th level spells.
I also need to think of capstone abilities. The evolutions are nice but they are a gradual increase along the course of the class. Right now I'm leaning toward making the astral avatar become more like an outsider, gaining outsider-like traits until finally becoming a native outsider like a monk.