Nobody knows for certain if the Outsider even exists. Legends say that in the Endless Void outside the Whorl, a space incomprehensibly vast that can be glimpsed through portals deep in the Underworld, lies a being that embodies the madness of the void, an entity of formless chaos so incomprehensible to the protected minds in the well-ordered Whorl that merely contemplating it will drive them mad.
If such a being exists, it lies outside the purview of the gods and beyond the order imposed by Vac, the hidden language of magic. If it doesn't exist, its legends and cults prove remarkably versatile and difficult to completely eradicate, no matter how hard the temples of the gods may try.
There are some particularly daring (some would say insane) warlocks who claim to have forged a pact with the Outsider. Their magic appears to work, and must have a source, but getting a straight answer out of someone who would risk their sanity for incomprehensible power is quite difficult. Many wizards and scholars would claim that these madmen simply channel more ordinary magic as it's understood in the Whorl, tapping into it in much the same intuitive way as a wild-magic sorcerer.
The truth about the Outsider remains, and will likely always remain, elusive.