The Cannibal Cults
First contact was made with this group during Operation 21 when we were called to assist Greyditch with thecult's assaults.
No lines of communication have been opened, the only contact has been violence, initiated by the cults.
What We Know
Medical scans show they are highly irradiated and have a far lower ambient body temperature, which means they are largely invisible to our drones.
It isn't confirmed but scans suggest they have evolved these traits over time, and they are potentially forcing evolution's hand by killing off those without them.
Tactically, they are guerrilas, hitting and running. When assaulting our base they fired, moved, and dug in to sniping positions. When attacking a mission party they used ambush tactics and variants on the hit and move seen at base.
They are highly adept at stealth, beyond what all but the FEF's very best can do. Tech is akin to tribal levels as far as firearms go, no armour has been noted.
A small proportion appear to be able to resurrect themselves once only. This is not regeneration, this is explicitly a return from death.
An assault on the base later in the second day revealed that a small number of the cannibals carried and used a collection of salves and several nanite patches (tox rems and basic med patches). The nanite patches are confirmed to be of Bunker Ascendant manufacture. Several of the patches had been used by the cannibals on each other.