Energy-based beings living in the nebula FGC-47, one of which takes the shape of a child's imaginary friend, Isabella, to judge whether the Enterprise-D crew deserve to live.
These lifeforms exist as small points of light among the strands of FGC-47, a nebula that has formed around a neutron star. They feed on energy, and a starship's shields and power systems are an attractive source for them.
They appear in the Next Generation episode Imaginary Friend. One enters the ship and reads the mind of Clara Sutter, a lonely girl, then becomes her invented playmate Isabella, visible at first only to Clara. Wandering the ship as a child, it is repeatedly told where it may not go and concludes that adults are cruel to children.
Isabella calls in others of her kind, which begin draining the shields. Picard explains to her that rules exist to keep children safe, not to hurt them. She relents, and the Enterprise leaves an energy beam for the beings to feed on.