I'm not terribly good with new diseases, but the core idea is as follows, in rough draft form. All names (including disease name and template name) are just placeholders until I come up with something more fitting:
Template: Fey Mad
Savage Maw (Ex) The afflicted gain bite attack dealing 1d4 + Strength (+ disease). Bleed 1 on critical hit (automatically infects). This attack is treated as a secondary attack when making a full attack.
Savage Resilience (Ex) For every two points of Intelligence lost in this fashion, increase the base creature's Strength and Constitution by 1 point each.
Glide (Su) You take no damage from falls (as if from feather fall). In addition, you can move up to 5 feet in any horizontal direction for every 1 foot you fall, at a speed of 60 feet per round. You cannot use this spell to actually gain height, merely coast in other directions as you fall. If subjected to a strong wind or any other effect that causes you to rise you can take advantage of it in order to increase the distance you can glide.
Fast Healing 1 As per universal monster rules.
Hide from Fey (Su) Creatures of the fey type and faerie subtype cannot see, hear, or smell the afflicted. Even extraordinary or supernatural sensory capabilities, such as blindsense, blindsight, scent, and tremorsense, cannot detect or locate such creatures. Non-intelligent creatures with the fey type are automatically affected and act as though the creature is not there. An intelligent fey creature gets a single Will saving throw (DC 10 + 1/2 the afflicted's character level or Hit Dice + the afflicted's Cha modifier). If it fails, the subject can't see the creature. If it has reason to believe unseen opponents are present, however, it can attempt to find or strike them.
Silence (Su) All sound by the afflicted is stopped: while the afflicted may appear to speak or growl, no noise whatsoever issues from the afflicted.
Notes: Considering adding in spider climb and nondetection, as well, as always on abilities.
Fey Madness
Type disease, contact, injury; Save Fortitude DC = 10 + 1/2 the afflicted's character level or Hit Dice + the afflicted's Cha modifier; in the case of contact, the DC of the save is 20.
Onset 1d6 days; FrequencY 1/day
Effect 1d3 Intelligence damage (this damage cannot be healed while the creature is infected); Cure 3 consecutive saves.
This disease reduces creatures with the fey type or faerie subtype to rage-maddened beasts, altering them physically as well as mentally. Those afflicted with the disease become chaotic evil after the first failed save and actively attempt to hide their affliction, going so far as to isolate themselves to allow the disease to run its full course. Any ceature of the fey type or faerie subtype reduced to 0 Intelligence while infected gains the Fey Mad template and is beyond the help of magic short of a miracle or wish. Non-fey are immune to the disease, instead becoming carriers.
Right now, it's a very rough idea and is intended, primarily, to be an issue for a fey and faerie (a racial subtype in my setting). While there are several types of faerie (Sidhe, Sith, Shifters, and so on), as a general rule, they get along, despite differing philosophies. The idea was inspired by Orthos' Shadow Curse as a "flaw" to the harmony of an otherwise flawless race (flawless, in their opinion, of course). Hide from Fey keeps them from just destroying each other if they meet, for example, so most of the abilities are for practical, as well as thematic reasons.
I'd appreciate in any help polishing this up, in keeping with the core concept, including CR adjustment advice, specifics on how a magical disease/curse should be removed, and so on.