A seemingly new power, connected with a place called by Nikolas `Le Salle de las Ombres' and by Andelia `The Hall of Doors', and with the Phoenix that Meredith became after her death as the Guisel Queen at Thalion's hand. Worm to Phoenix, as Raven's prophecy said. The Phoenix is, seemingly, the physical incarnation of the Void, as the Unicorn is the incarnation of Order and the Serpent the incarnation of Chaos. Void is sometimes, incorrectly, called Abyss, because it is related to the Abyss in Chaos, but the Abyss is merely an aspect of the Void, a place where the power of the Void is very evident.
The one generally known ability that this power gives is an ability to open a door to somewhere other than what the door normally opens upon. The Void initiate can instead open the door to the Hall of Doors and enter it, proceeding along the hallway to their destination and exiting a door, perhaps the door they entered, or perhaps another. Where the Mirrorrealm is composed of mirrors looking out into the world, the Hall of Doors is composed of doors opening onto the world.
How one becomes a Void initiate is somewhat of a mystery, as the one known initiate (Andelia) has never been asked how she learned the power. Perhaps there is something equivalent to the Pattern or the Logrus hidden deep in the Abyss that she traversed, or perhaps it is more mundane, or more mystical.