The short version is that the Venatori are a group of werewolves slightly different from the Normal Uratha of the World of Darkness. They carry a mystical organism/substance within their blood that causes their werewolf genes to express themselves in an altered form.
When they first transform into Venatori, each of them is bonded to a mysterious spiritual entity (known as the Qashmallim; a process which inhibits their ability to learn the gifts of the normal Uratha, but provides them with a link to the Supernal Realms (the source from which awakened mages drawn their power).
Because the symbiotic ‘virus’ which activates their wolf-blooded genes into this alternate configuration is carried in their blood, the Venatori have the potential to convert wolf-blooded individuals (including full Uratha) into Venatori (a transformation which must be willed by the consciousness of the individual in question).
As a faction, the Venatori are investigators and monster hunters. They see it as their duty to protect the world from supernatural threats.
…and the Long Version
The Venatori (singular: venator; the Venatori are also occasionally known as Hexenwulf) are werewolves, cousins of the Uratha (the more commonly known werewolves in the World of Darkness). They are the bearers of a separate strain of lycanthropy, distinct but related.
A Venator is a hybrid being, a creature of the physical world empowered by spiritual energy and an innate connection to the supernal realms. They blend human and wolf, spirit and flesh, conscience and rage, and possess both magical and physical prowess.
The Venatori see themselves and as ancient and secret organisation (a ‘benign conspiracy’) with a duty and mandate to act as the supernatural guardians of reality and humanity. Consequently, their self-proclaimed mission is to investigate (and where necessary, oppose) the eldritch, eerie, esoteric, and enigmatic.
The organisation has existed in one form or another for centuries, and has friends in high (and low) places; it even operates with the tacit approval of certain key figures in various governments around the world (since the Venatori as a whole fall on the pragmatic end of the spectrum on the issue of the Masquerade, and secrecy from humanity).
The Venators employ a cell-like structure of individual units (packs) that initially have only a few links to the rest of the order members.
The division of the organisation that the PCs will work for arose in the Victorian era, and initially had unofficial links with the UK government. These links became more tenuous over time as the venators faded into the background, and during the Edwardian era the last significant ties (siphoned funding etc) were cut.
Those who posses the werewolf gene, even in its dormant form, display a deep (and dangerous) instinctive inquisitiveness and urge to investigate mysteries. Once their supernatural powers are activated, these instincts grow and sharpen into an ever-present urge to hunt. Venatori can actually draw essence from the consumption of supernatural prey, and thus they tend to develop a taste (and hunger) for paranormal energies; this is one of the more disconcerting traits that a new Venator can find that she/he posseses.
According to Venatori scholars, the mysterious genes possessed by the ‘wolf-blooded’ population that sometimes express themselves as full lycanthropy (if the individual experiences the ‘First Change’ that unlocks their heritage) also have a second form that expresses only when combined with a separate (and rare) viral strain (although the mystical heritage of the bloodline is somewhat more esoteric than mundane genetics or virology can describe). This ‘Venatori strain’ appears to have existed for thousands of years, and can lie dormant and recessive for generations before activating in response to the presence of supernatural energies (leading some Venatori to consider themselves to the the immune system of humanity, antibodies that awaken to combat paranormal ‘infections’).
The consequences of all this are that, although some rare wolf-blooded individuals possess active traces of the Venatori virus in their bloodstream naturally, most Venatori come to be when a wolf-blooded individual (sometimes including full Uratha, although this is uncommon) are infected with the viral strain. Wolf-bloods generally posses very robust immune systems, so it usually takes a decision (either conscious or unconscious) on the part of the host to ‘succumb’ to the virus.
In addition to activating naturally, the Venatori gene/viral strain can also be awoken by an infusion of essence (and a dot of willpower) from another Venator (often received in saliva in a bite of kiss, or in a sip of blood, a-la vampire/werewolf myth). The characters may have been ‘recruited’ in this way, or awoken naturally.
When the werewolf genes of the host are unlocked/awakened by the active viral strain (a process that is poorly understood, even by Venatori scholars), the individual becomes bonded with a strange gestalt spiritual entity (known as the Qashmallim, amongst other names), which is supposedly an incarnation of divine fire, or a host of angels… or a primordial god (or all manner or other things depending on who you ask).
A true Venator must bear an awakened Venatori gene, but the organisation does occasionally include talented mortals (and other supernaturals) as well. These are never given access to the full secrets of the venatori (as much for their own safety and sanity as anything else), but they are guarded closely, as a valuable resource that needs defending.
The Venatori are hunters without parallel (indeed, their very name originated with the Latin word for ‘hunters’). In addition to their shape-changing and regeneration abilities, they possess phenomenal strength, speed and stamina.
The Venatori use many of the same rituals and rites as other werewolves, but they don’t have the same affinity for spirit powers as their Uratha brethren (the energy of the divine fire bonded to their souls causes interference with spirit numina, making it difficult to learn gifts). Instead they have access to the arcane powers of the supernal realms through their link with the Qashmallim, enabling them to wield magic similar to that of awakened mages.
Unsurprisingly (given their shared heritage) most of the rote spells taught by the Venatori bear a striking resemblance to the gifts commonly found amongst the Uratha.
Just like their cousins the Uratha, the Venatori have 5 forms, and can shift between them at will: human (hishu), near-human (dalu), war-form (gauru), dire-wolf (urshul), and wolf (urhan). The Gauru form is taken only for battle, as it is a strain to maintain and is capable only of dealing death and destruction (which it excels at).
In addition to shape-shifting they can regenerate most damage at a massive rate, but they have weaknesses…
Silver is a bane to them (causing horrific wounds that take ages to heal), and at all times they must maintain control over the Rage that is present in their spirit, particularly when angry or when in the war form.
The existence of werewolves must be kept secret- a natural defence mechanism which facilitates this is Lunacy, a supernatural aura of terror and confusion that they emanate when in their hybrid forms (particularly in Gauru form).
Venatori experience the same problems with silver, rage and lunacy as the Uratha; in system terms, the only singnificant differences between them are that they have arcana and rotes rather than auspices and gifts.
Special effects note: in addition to the silvery moonlight effects of renown, Venatori have a Doctor Who/Tardis/time vortex energy effect (because it evokes the spirit of light and fire that they are bound to, and because it looks awesome).
In early drafts the Venatori were known as the hounds of Helios or the lodge of the ascending flame.