Harbinger

The last gasp of the Primordials, the twelve Harbingers are pure manifestation of their primal desire to escape the prisons they have been chained in. Much like an Edict that channel The Powers, the Harbingers are creatures immense power and guile. Each serving a separate Primordial, all with the intent of freeing them from their prisons.
Each Harbinger is trapped in a Place of Power such as the Frigden or Hellfire Gorge.
Individual Harbingers are fundamentally different, each however retains a horrific visage and possesses some sort of elemental power with the following being seen. 
Alogorth: A horned creature, towering over 12ft. In height. It seems to possess great destructive powers and the ability to control all of the elemental forces. Can control and summon elementals. Is rumoured to be a great strategist. Currently trapped in Hellfire Gorge.
The Keeper: A four armed, blind creature which always keeps two cages on it's person. It seems to be able to trap souls within the cages and use the powers that those trapped souls once possessed.  Sealed in the Boreal Shelf, it has the power to make whatever it whispers into a mind become truth. 
Venbria: A winged beauty, who has the ability to read minds and to shift her form. She is also known as she is a master of deceit and lies, known as the Many Faced One, She Who Waits and The Cunning Queen. Tricking the Genasi and The Yushan Empire, she escaped from the Frigden, she was instrumental in freezing the Frozen Sea. She has since disappeared, likely plotting via one of it's guises.
Toriel: A Massive bull like creature, seeming unstoppable. It can destroy near anything and controls the powers of thunder, lightning and fire. It still roams somewhere in the Glass Ocean, seeking to find the power to fully open the rifted prison
The Silent Twin: Trapped in the Helix Tree, little is known of this harbinger, only that it yearns to be free.