Undated_Abyss
Lore-wise the largest threat to existence would be the fragment of the Primordeal that slumbers beyond the fabric of reality, however, in order to even awakening that, someone would have to break large parts of the time-space apart and attempts to damage time are currently met with strong aggression from the Judges.
However, none of these have any actual faction or ships currently nor does any information about them exist and only three factions on the entire forum know about the Judges while none of them know about the fragment.
Aswell, logically it is ill-advised to damage time-space as it contains large parts of this reality-shards stability.
While, considering forum posts I´d probably guess for the Ancient Army.