The Children of the Storm are a non-corporeal species native to the Delta Quadrant. They can travel individually within energy shells operated by thought, within which they exercise only one distinct skill set each (e.g. communications, weapons, command), reflected in their resonance frequency and the color they display.
The Children of the Storm originated as "thoughts" from a gaseous living world, referred to as their "mother" by the USS Voyager crew. The mother of the Children was countless centuries old when she was discovered by the Voyager crew in 2381. Having witnessed unparalleled destruction by the Borg for resources, the mother came to highly prize the plant-based lifeforms destroyed by the Borg. At some point after her encounters with the Borg, the mother withdrew into a dying star system, leaving her Children isolated and cut off from her.
The USS Aventine made first contact with the Children of the Storm in early 2381 during exploratory runs to the Delta Quadrant, during the Borg invasion of that year. At the periphery of their star system, the Aventine crew witnessed a graveyard of Borg debris, suggesting the ability to destroy on an unprecedented scale. By possessing a Ullian member of the Aventine crew, the Children expressed a xenophobic aversion to future contact with the Federation. (ST - Destiny novel: Mere Mortals)
By late 2381, once the Project Full Circle fleet had arrived in the Delta Quadrant, three vessels were dispatched to follow-up on the Aventine's discovery in hopes of better understanding a species capable of wreaking such destruction upon the Borg. However, the investigation soon went awry, resulting in the destruction of one vessel, the near-destruction of another, and the capture of a third by the Children.
The efforts of the USS Voyager and Commander Liam O'Donnell of the USS Demeter managed to salvage the relationship between the Federation and the Children of the Storm, as well as, finally reunite the Children with their lost mother.