"Adjutant" is the Cerberus codename for the experimental subjects created at a secret facility beyond the Omega-4 relay. The creatures are based on Reaper technology, and the adjutants encountered on Omega have the ability to infect any life-form to create a new adjutant.
Adjutants are deadly pack hunters. They initially remain hidden, observing potential victims and distracting them with subtle whispers. When they select a target, adjutants ambush them with charged particles that create a violently unstable mass effect field that causes severe disorientation and renders the target helpless. Enhanced muscle tissue gives adjutants the ability to leap surprising distances, allowing them to close in and infect their target quickly.
The baseline form of the being does not match any known species. Xenobiologists suspect that Reapers may have originally created and implemented the virus several cycles before the Prothean extinction and that it still carries genetic traces of its original host. If this is true, then like the Collectors, adjutants are a glimpse into the distant past at a species whose very existence--and struggle against the Reapers--has long been forgotten.