The Dark Armies are Unicron-allied evil beings of some sort in the Generation One continuity family.

Who or what they were remains unknown but in the first telling of the Primus/Unicron myth by Unicron himself mentions fighting alongside his Dark Armies, back before he was trapped in his physical form. It is possible they were other dark gods like himself, since another early version of the myth mentions him as being part of a pantheon, or they could just have been lesser beings in his thrall.