Timeline:
Begins: Summer before Fourth Year.
Ending: Whenever.

Alterations to Canon:
None, beyond fanon tropes.

Pairings:
Harry/OFC!Succubus
Harry/Hermione
Harry/Harem
Suggestions: Lavender, Parvati, Katie, Luna, Ginny, Daphne, Susan, Tonks, Fluer.etc

Story:
By accident Harry picks up more than he bargained for when he buys a new used school trunk from a store in Diagon Alley the summer before the Triwizard Tournament. In a hidden compartment is a magical artefact that contains the bound spirit of a Succubus, trapped several thousand years ago by a Chaldean magician. Having served as the counsel, servant and concubine to several magical kings, sorcerers and scholars over her many centuries she possesses a great deal of valuable magical wisdom she is more than willing to share with her new host/patron.

There's a catch though. She can only manifest herself and communicate with Harry for brief periods without nourishment and for a Succubus that means sex magic/energy. She can feed a little from Harry's aura but its the blending of male and female sexual magics which create the energy Succubi and Incubi need and in ages past fed upon humans to gather. In order to feed her long enough to teach him Harry has to have intimate relations (of differing degrees, the greater the intimacy and release, the greater the nourishment) so that she can feed off of it.

Harry introduces his new ally to Hermione who, enticed with her almost perverse thrill at learning ancient/lost/powerful magicks as well as acting on naughty fantasies she has had about her best fiend, is promptly 'seduced' to the cause. After which Hermione aids Harry in gathering other witches to the task, each new 'participant/donor' enhancing the potency of Harry's sex energy. She also teaches Harry the finer points of pleasuring a woman (several thousand years worth of experience) making Harry an accomplished lover to his growing harem.

With the addition of more witches Harry's magical strength grows, he performs rituals to amplify and project his aura, learns how to dream walk (a talent of Succubi) into the minds of witches whose dreams feature him, how to convert the potential procreative power of sex into potent magical fuel to raise wards, enchant, heal and other disciplines.

With her counsel and using the Succubus' esoteric knowledge Harry is able to determine the greatest benefits he can gain from his unexpected entry into the Triwizard as well as how to confront the tasks. He takes the tournament by storm, drastically changing his reputation amongst his peers, the visitors and the general population of Prophet-readers.

Additional information:
Choose your own name for the Succubus. Preferably one reflecting her ancient origins, perhaps a historically significant name or she might even let Harry choose what to call her based on a litany of names she has been known by during her life.

Suggestions: Astarte, Ishtar, Shaushka, Anahita, Baphomet, Meridiana or Qetesh.
Most of them are goddesses or demi-goddesses from ancient polytheistic pantheons, often known for their fertility powers. Baphomet being the 'satanic' figure purportedly worshipped in secret by the Knights Templar and Meridiana said to be the name of a demon who came to a man named 'Gerbert of Aurillac' and helped him ascend to the papacy as Pope Sylvester II.

So if you're going to give her a name that seems/sounds like a modern girl's name, at least excuse it by having it be a shortened/derived name from a full older/ancient one, i.e. 'Tesh', 'Diana' or something like.

The Succubus' appearance is up to the author too. Red all over and horns on top? If you wish I guess. A fallen angel-esque motif, featherless dark wings (typically not outwardly expressed?) with dark or red eyes? I had this vague idea/amusement to imagine 'Trance Gemini' (in her purple toned phase, complete with prehensile spade-ended tale, from the first season of Gene Roddenberry's 'Andromeda'), go ahead and Google it if you like.

The Succubus' Powers/Gifts should AT LEAST include:
- Dreamwalking. The ability to enter aroused/erotic dreams, witness them, manipulate them and substitute/impersonate a person in a dream. A limited skill she can pass to Harry, so he can visit girls who have sex/dream fantasies about him as they sleep.

- Allure. Not totally akin to that of a Veela, but within her host, Harry, she's able to push out, through his magical aura, a magical compulsion increasing arousal and lowering inhibitions but not necessarily targeting that arousal, desire or disinhibition towards Harry himself.

- Pleasure Touch. The ability to channel a derivative of the Allure compulsion through skin to skin contact to directly stimulate pleasure. 'Magic Hands' so to speak, any slow carress or prolonged holding of hands stimulating a witch's arousal powerfully.

- Glamour/Shaping. The ability to create very strong, powerful and flexible glamours that draw their form from the subconscious of her host (her initial first appearance to Harry) or others (when Dreamwalking) and a skill she can teach Harry. Over a prolonged period she can even permanently reshape/transform the substance of Harry's body (if she has sufficient nourishment from Harry's sexual activity), allowing him to reverse malnurishment-induced stunted growth, 'enhance' certain attributes, correct his vision. Also a talent she can work on witches who have enough of Harry's sexual energy in their bodies, witches in Harry's harem able to gradually alter their figures, shape, bust, height, hair length and texture.etc.

From the outset Hermione ought to be bisexual, perhaps having already watched Lavender and Parvati playing with each other (with Lavender and Parvati knowing/inviting Hermione to watch). This leads to Hermione looking to recruit the two of them to Harry's and the Succubus' needs. Each added witch begins looking about for a new recruit to groom/mold for eventual membership.

Fleur should be hypersensitive to the Allure that Harry generates because of the Succubus and her sexual energy should be incredibly nourishing for the Succubus. Likewise Tonks' metamorph status makes her sexual energy incredibly adaptable and thus as potent, when mixed with Harry's, as that gather from any several other witches - and having Harry's Succubus augment sexual energy in her body drastically elevates Tonks' magical strength and morphing capabilities.

Feel free to diversify some of the kinks amongst the many witches that join Harry's harem. Lavender likes having her hair pulled? Luna likes to do it au naturale outdoors? Katie likes dirty talk borderline porn speech? Fleur likes having her nipples abused? Hermione is absolutely determined to lose her gag reflex? Susan loves anal? Daphne enjoys tit-shags? Cho loves to watch Harry do other witches? Parvati likes to be restrained? Ginny likes to be spanked? Tracy Davis enjoys facials? Tonks enjoys a bit of dom/sub games with other girls?