A Skrill was a genetically engineered species that served as implants to Humans that served the Taelons.
Originally, they were an sentient alien species that were encountered by the Companions at some point in the distant past. This initial contact was a disaster and the Skrills attacked the Taelons. Despite their destructive power, the Skrills were eventually enslaved and taken aboard the Taelon mothership. When the race arrived on the planet Earth, these early Alpha strain Skrill were given to Human bio-engineers in order to turn them into a weapon. Using Taelon genetic engineering, they were able to turn the species into a breed of weapon symbiotes and managed to develop the more docile Gamma breed of Skrill for Companion agents. (Episode: Scorpion's Dream)
These creatures bonded to the central nervous system to a host body where they served as another appendage to the organism it had symbiotically connected to during the union. They were controlled through a CVI implant located in the brain of a host. During the intial part of the bonding, there was some discomfort felt by the host. After placed on the arm, the tentacle legs of the creature bore through the skin of the host body thus sealing the two into a symbiotic pact. Once completed, they served as a living weapon capable of firing powerful energy blasts at even distant targets. The power of the energy attacks were largely derived from the emotions generated by the host's mind. (Episode: Decision)
Those that possessed Gamma breed Skrill typically were considered in a true symbiosis with the Skrill. The symbiotic implants drew nourishment from their hosts and used their electrical field to radiate their own. In turn, they gave their hosts powerful energy weapons to use at their disposal. In time, the creatures own mind also intruded upon the hosts which were actually a sort of communication. The images they showed their user were primitive and irrational but vivid. This was because the Skrills possessed their own intelligence as well as personalities and one could not treat them as a device but rather a partner. Thus, their hosts tended to treat the creatures as a relationship and had to form a true means of communication with them. (Episode: Scorpion's Dream)
At the time of a public speech by the Companion Da'an, a sniper attempted to kill the Taelon. The initial attack failed and Ronald Sandoval used his Skrill to fire an energy attack at the snipers located. After a CVI was implanted in newly recruited Protector William Boone, he was taken before Da'an where he recieved his Skrill weapon which attached itself to his arm. (Episode: Decision) The insane murderer James Pike after his CVI implant malfuncioned saught to later kill Da'an. To accomplish this feat, he had his implant altered and later stole Ronald Sandoval's Skrill in order to use the weapon to kill the North American Companion. (Episode: Avatar)
Later when Da'an devolved into an Atavus, Zo'or had Liam Kincaid and Lili Marquette being accompanied by Companion Protectors as only their Skrills were powerful enough to harm the creature. (Episode: Atavus)