The Voice is a figure only known and visible to Titans. It is what allows previous Titans to speak to the current ones. Although it is dangerous, as using the Voice can put one to sleep for several days. If they use the Voice and stay asleep for 10 days, then the user will die. Enabling this magic can only be used by trained Titans. This can also enable Titans to view previous and other Titan's memories.
It is called the Voice for one reason alone: because voices of spirits rule the Voice. The Kielyn Side is where the melevolent spirits rule. However they cannot cross into the lighter side, they can speak to people who are one that side of the plane.