I would like to run a Prototype kind of monster (from the games of the same name in XBox 360). One that revolves around being a parasite to a human host that gives it all sorts of evolved powers at the expense of forcing the host to (eventually) serve an ulterior purpose. The biggest difference would be that I would like the parasite to be plant based rather than aberration based. The idea is that one of those alien Moonflowers swallowed a plant enthusiast alchemist (think poison ivy prior to being greenified) who instead of being turned into some kind of clone was instead implanted with a parasite. The idea being that whatever (whoever) sent the Moonflowers to Galorion would need an agent that could blend in with the natives.
That is the biggest difference. Where it gets similar is that the host would not be immediately aware that he is harboring this parasite within him. When he does figure it out, he does not know its true nature, much like the protagonist in the game. Game wise, it would work exactly like the Gillman trait "Servitor" which means there are key triggers that will force him to act in a certain way or to a specific goal.
The shape shifting protagonist had many strange weapons at his disposal, some of which can be easily duplicated in the eidolon evolutions alone:
claws = claws
hammerfist = slam with improved damage
whipfist = tentacle with reach x2
Other abilities that are easily replicated in the advanced race guide or eidolon evolutions:
Climb
Glide
fast healing (although limited in amount of damage healed or circumstances in which he heals, like needing direct sunlight or limited to half his hitpoints)
unnatural
vulnerability (in the game it was a kind of anti-virus, but that can be easily replaced with fire or some other element poisonous only to plants)
In the newest game, the protagonist had another tentacle attack that entangled and ripped apart enemies in a really dramatic fashion. The closest equivalent I could find was the "Entrap" UMA and tie it to a shorter range tenticle/slam attack (or require multiple hits). It doesn't rip apart anything and depending on the DC, it could be difficult to entangle anything but if it works (twice in a row) then it is a show stopper.
The shape shifting aspect of the game was probably my favorite bit as it allowed the protagonist to assume the shape of nearly any humanoid he had encountered (and consumed) and pass off as them, adding a great stealth element to the character. What was even more interesting is that at times it was clear he didn't have quite a lot of control over it as sometimes his form would shift and churn into weird shapes or colors and his voice would revert back to his own when he wasn't paying attention to it. Thankfully, Pathfinder has an ability just like that with its "alter self" spells and spell-like abilities (though you would have to add the whole "consuming" part to it).
Another convenience is that the eidolons have a limited number of natural attacks allowed that will gradually increase over time. This means that much like the protagonist in the game, he will have to gradually evolve his monstrous abilities rather than get it all in one giant, disgusting package.
<Backstory> Bramble Brewers are alchemists who have an affinity with plant life - both in its growth and its destruction. However, their training can be quite hazardous if the pupil doesn't show any natural talent. So it was for the half-elf, Jeram, who found his aptitude for the art did not match his fervor. Traveling with his master to all of the exotic jungles and forests of the world, he was often regulated to talking mule or memo taker. Far be it for him to learn something by having it dictated to him, he often decided to go out on late night "walks" and gather some plants and knowledge on his own. One such night while traveling in the untamed jungles of the Mwangi Expanse proved to be his undoing. It is there that he encountered a copse of breath taking flowers with an unearthly glow. His master would have been able to immediately identify their danger but the apprentice had no such experience or knowledge. He walked into that jungle during a sweaty spring night. It wasn't until a heat stroke inducing summer that he finally walked out. His rags for clothes and a single sling backpack with his name on it being his only worldly possessions. He feels tired and confused most of the time he is awake, and he has strange and vivid nightmares of even stranger jungles in his sleeping hours. Even on nights when he goes without sleep at all, he still falls into a dazed as the vision of the strange jungle realm enters his mind. Thankfully those events are becoming more rare but behind the smiling face and cheerful demeanor, Jarem knows that there is something wrong with him and as he makes travel back to more civilized lands, he finds himself unable to remember of the exact details of how he got there or where he is going. <Backstory end>
