Through the MAW
The Roil is a Plane of Energy that runs parallel with humanities Plane of existence. It was first theorized during the creation of MAW technology and later confirmed during the Strange Year as the denizens that reside in it poured out of permanent MAW portals and attacked earth (also, depending on who you ask, confirmed by Devon Farngill).
According the the Devotions of the Six it is a realm of deities, angels and demons, lost souls and chaos incarnate. To see it with unshielded eyes is often a sentence of insanity as the mortal mind cannot truly comprehend it. Thankfully, traversing the Roil through a MAW is near instantaneous not giving travellers time to view it’s glory.
The Devotions of the Six teach that the Six live and reign there, however there are ‘others’ present that they have battled throughout the span of time. The denizens of the Roil are created and used, to a limit, by these deities, most being given autonomy to spread their particular masters will. Braching the Roil, with a MAW or other means creates a beacon in the Plane, attracting any and all nearby denizens, daring them to escape the confines of their world and unleash themselves onto a new Plane.
The Roils energy is able to be channeled and bound to sentient will, as seen by human Roil Channelers and Fractals Roilers to create dramatic and extensive effects, from raining down hellfire, to healing, to purposefully summoning a creature from the Roil. Humankind is still in its Roil Channeling infancy, most Channelers dedicating themselves to learning only a few techniques, but learning them well as improper use as well as channeling to much Roil energy often leads to disastrous effects.