The Horla is a creature not well known outside of the fans of Vincent Price and die hard fans of weird fiction who have read the original short story, which created the entity. The Horla is an extra planar entity which first appeared in a short story named after the creature itself. The story "The Horla" was written by Guy de Maupassant in 1887. The story served as an inspiration to H.P. Lovecraft who, in 1927, interpreted it thus:
"Relating the advent in France of an invisible being who lives on water and milk, sways the minds of others, and seems to be the vanguard of a horde of extra-terrestrial organisms arrived on earth to subjugate and overwhelm mankind, this tense narrative is perhaps without peer in its particular department."
The story later served as one of three inspirations (all three being Maupassant stories) for the 1963 Vincent Price film "Diary of a Madman." The entity is changed a bit in the film, and I will used what I believe to be the best aspects of the Horla from both the short story and the film.
HD 2; AC -1; Atk Slam 1d4 or by weapon; Move 12; Save 16; AL C; CL/XP 9/1,100; Special: invisibility, possession, obsession, limited telekinesis
The horla are a race of extra planar (or extra terrestrial) entities bent upon the domination of chaos and evil over the peoples of the world. They achieve this goal through the haunting of individual hosts while manifest upon the material plane, causing their hosts to commit unspeakable acts. When these entities manifest upon the material plane they live upon water and milk, though it is unclear as to why these are the substances which support them. There seems to be some sort of law in effect which allows the horla to only acquire a host in one of two ways. The first is for the host to hail or summon the horla in some way. This can even be an accident (waving at a vessel the horla is on, saying "hello" into the darkness to see if someone is there etc.) The second way is to come into physical contact with someone being haunted by a horla, violently or otherwise. At this time the horla can switch hosts.
A horla, in it's physical form, is always invisible. Spells which cause invisible creatures to be seen are of no effect. The creature can only be seen if something covers it. All horla have minor telekinetic abilities. They can move relatively heavy objects with their minds, but not with much force. A large urn could be pushed from a pedestal, doors could be slammed, but a weapon cannot be properly wielded. The horla can, however attempt to use it's host to achieve its evil ends. The horla will begin it's haunting by attempting to play on it's host's desires through conversation and an attempt to drive the host mad to obsession. If this is unsuccessful, the horla will outright possess the body of the host. Rules on both obsession and possession can be found in this post. The horla will always attempt to corrupt and cause obsession in a host first, preferring to cause the host to turn to chaos and evil itself.
In regions where the horla are known, it is rumored that cults of chaos and horla worship can be found. The members of these cults actually seeking to be hosts to the horla, that their evil ends might come to fruition.