“We are the Phantomsssss blesssed by the Nyte... We are harbingerssss of end timessss”-A unnamed Phantom Harbinger

Phantom Harbingers are the basic soldiers of the Phantom Legion.
Before they were purple slicey dicey killing machines they were once Demons, mostly Hatelings.

When Aka Manah became the Phantom Beast, he looked for a more powerful army to seek revenge with, which is when he turned to the Nyte. Nyte is a chemical only found within the Phantom Realm which is able to change a organisms physical appearance and strength, sort of like the g virus from the Resident Evil series.
But instead of turning them into gross anus monsters they turned them into what resembles Phantoms, though we do not yet know what a full Phantom truly looks like

Once the Phantom Beast got his hand on Nyte he went batsh*t crazy with it and started dumping any living organism he could find in a pool of Nyte to see what nightmare concoction he could come up with.
Which then led up to him subjecting some of his followers(some idiot Demons) to the Nyte, which led to the Harbingers.