The Void between Gates is another plane of reality. When traveling through a Gate, a person passes momentarily through the Void, which is how the spell works to eliminate geographic distance in reality. That brief moment of disorientation a person experiences is the moment in the Void.
The Void is part of the cycle of magical energy as it moves through the planes. This property was particularly important when dealing with the present-day Mage Storms. Using Karal and one of Urtho's weapons, the mages channeled the magical energy of the final Mage Storm, an echo of the original Cataclysm, into the Void, using it as an energy sink.
The Void was useful to Ma'ar as well. He built a sanctuary in the Void to house his spirit. Whenever his body was killed, the energy released by his violent death catapulted his spirit into its shelter in the Void. There it remained until a suitable host body among his descendants was available to invade and possess. Tucking his spirit away in the Void allowed him to wait generations until he reemerged.
His last incarnation, Mornelithe Falconsbane was lost in the Void after his defeat near k'Sheyna Vale. It was believed he was dead, however, Ancar managed to accidentally pull him from the Void with a botched Gate spell. His second defeat launched his spirit toward his sanctuary, but this time Firesong was ready. The Healing-Adept tore open the sanctuary and shredded Ma'ar's spirit, breaking it down into tiny bits of energy and scattering the pieces through the Void.
Later, with the Mage Storms eroding his personality and mental processes, Firesong began journeying into the Void. He closely examined the sanctuary and the spell Ma'ar had used with the idea to use a modified version of it himself. Once the Mage Storms were no longer affecting him, he gave up the idea.