One of the parasites
A lifeform unlike any other, and arguably the nemesis of CDN, if the Reapers (pre-Control) don't count.
QoroQ was (is? Hopefully was) an abomination. The creature defies scientific classification, and is capable of multiplying and assuming many forms. Perhaps its most horrific ability is to form parasites capable of infecting and possessing a host’s body.
The possession, however, is only the first stage of infection. Given enough time, the parasite will eventually mutate its host into a hostile predator, capable of shapeshifting at will. The creature’s apparent ultimate goal is to remove all other forms of sapient life, perhaps the totality of life itself, and replace it with only QoroQ.
Saga[]
See:
(QoroQ was the primary villain of these early arcs, and fallout from the fight against him affected various characters for years afterward.)
Aftermath:
And They All Lived Happily Ever After: Dr. Fell and SCIENCE, er, triumph?
Whatever Happened To The Master Race?: The Fallen in the last days of the Reaper War.
Later Years:
Despite the events of QoroQ's campaign being made public, a surprising number of people regularly fail to remember what it was, or express confusion over an "advertising campaign", etc. Then again, CDN's possibly-STG moderators (in-universe) are censoring images of QoroQ, as seen here (no, Juhani and co weren't actually posting hanar porn...). Here's one from Branka, since the STG can't reach us here:
Art by Armax_Hammer
Juhani continues to worry about it here.