The Magnus Foundation arose in the late 1800s as the idea of giving supernatural creatures formal citizenship and rights under the the mundane legal system began to enter the wider discourse of the supernatural world. See, some supernaturals, such as vampires and dragons, were quite adept at blending in to the human world, and managed to gain wealth and influence in the mundane system, and the protections thereof. However, others, such as natural-born lycanthropes, spirits, giants, ghouls, and the created were often considered to be stateless beings, belonging to no nation in particular and thus not falling under the protection of the mundane laws. Likewise, fey, jinn, and horrors were all considered to be citizens of their own home nations--such as the Summer Kingdom, Jinnistan, or the Kingdom Over Shadows, and unless their particular nation had relations with a mundane nation, their citizens could find themselves effectively stateless and without protection.
The Magnus Foundation arose initially as an advocacy group to help protect the rights of stateless supernaturals. However, over the years, it evolved from a single law office in London into a network of sanctuaries and shelters for vulnerable members of the supernatural community--those whom regular shelters might not have the resources, expertise, or capability to care for. Homeless supernaturals, those recently affected by a transformation and thus not in full control of their abilities, and strange creatures with unique needs all find succour at the gates of the Magnus Foundation's network.
Agents of the Magnus Foundation continue to advocate for supernatural rights in mundane courts around the world, and their field agents scour the world for unique and special beings who could use their protection. They stand opposed to many of secret societies, who they feel are more interested in power than the common good of all sapient creatures. Thankfully, they've worked well with the Veil Treaty, and none of the major powers are willing to jeopardize the Magus Foundation's status as a neutral safe haven in the Secret War--even when the Foundation's agents sometimes steal an artifact or creature right from under the noses of the bickering factions.