The Harbingers

Harbingers are vessels which house the reborn souls of the Old Gods. Called gods in early eras of the universe, their role in society has shifted over the years, from feared rulers, to weapons, to prisoners. Several planetary systems have Harbingers, most notably the Sol system, where Earth houses many. Feared across the ages, their true forms are said to be something of a dream...or rather, a nightmare. For mostly that reason, they are outcasts and experimented on in the current age.

Causes

The Harbingers emerge, are reborn again and again, because the soul of a god so powerful that it transcends lifetimes. It is unkillable, so it must be born again in a living body. Why the souls of the Old Gods are tethered to the mortal world is also unknown, or why their true nature reveals itself so suddenly. 
There does not seem to be any reason why a certain Harbinger is reborn in a certain person, as there is no correlation between the past lives outside of containing the soul of a god. In the earliest eras, the Harbingers could more easily remember their past lives and more closely resemble their original self. As they die and are reborn again and again, they get more of a fresh start. Each life can have vastly different personalities, each having their own lived experiences.

Symptoms

The signs can emerge at any time, but typically surface during adolescence or any other life altering time. 
The Harbinger will first begin to dream of their past lives. They are described as lifelike yet surreal. They can have these dreams nightly for days or years before the other signs emerge. 
The next stage is unexplained magic. Either the Harbinger suddenly exhibits magical potential when there was none previously, or they experience new or enhanced magics out of nowhere. 
With the introduction of new magic, the Harbinger's tell-tale sign emerges: the Mark of the Harbinger. This mark resembles a small black tattoo that can appear anywhere on the body.  It is the about same with each reincarnation, so it is useful to identify the Old Gods in their new bodies. This is usually the symptom that gets the Harbinger noticed. 
From there, the Harbinger will start to feel a compulsion towards their divine form. They can tap into a pool of energy in something resembling their original form, but only for a short period of time. This is known as Transcendence. It gives them tremendous power, but it is dangerous to wield. 
The Harbinger will continue to grow in strength the more they interact with their past lives and divine form. While they can extend the period of time they can be in their divine form, it can never be permanent. Those who have tried to stay in that form for too long have died, and were reborn again.

Type

Divine