Commonly known as Extradimensionals, the Unbidden and its other two factions: the Vehement, and the Aberrant, are a sapient race made out pure energy.
These extradimensionals are a very aggressive race, choosing to invade galaxies known to use jump drive technology. They invade galaxies through a crack in time and space effectively creating a portal or bridge from their dimension to ours. They then would proceed to consume all life within that galaxy. Using strange see-through ships made out of pure energy, they search for life bearing worlds and begin to use their disintegration rays to suck the life force from the planet.To talk to a unbidden and its constituents is like climbing a volcano, there's no point. Even if a species successfully received a hail signature from one of their ships, the only response they would receive is "food" right before they are disintegrated.
Despite being a seemingly unified wrecking ball there seems to some sort of major infighting amongst their race. The vehement and the aberrant are hostile towards each other but also attack the unbidden ships when crossing paths. These factions probably fight each other over the vast "feeding grounds" and resources they come across.