The Vimans (once called the Gikkizzia by the Pure Combines ) were a species that had a powerful yet peaceful empire that ruled over 13 galaxies up until they were conquered by the Empire of the Combine Race over a Centrillion or so years ago of which much about the Viman-Combine War is mostly unknown with only a handful of information given by The Old One about the war as well as other information about the Vimans before they were enslaved. During the war, they were mostly known as the Combine Advisors and were known as "Shu'ulathoi" by the Vortigaunts , they were speculated to be the masterminds behind the Combine occupation of Earth, and were commonly speculated as being the master race of the entire Combine empire, although, this was proven wrong when the Pure Combines begin to fight (or rather lead) in the war.
During the Combine Enslavement
During the Combine Wars of Conquest and Enslavement
During the First Multiverse War
Golmar Earth Uprising
The Advisor orders Breen to teleport using the Dark Fusion Reactor of the Citadel to enter their own universe, where it demands Breen be put in a host body (presumably another Advisor) to survive. The Advisor then gains control of the computers and signs off from the screen, preventing Alyx from gaining control when she tries to stop the teleportation.
When Gordon and Alyx head back to the Citadel, a recording of Breen is seen from the Advisor's point of view, at which the creature's face appears all over the screens and a real Advisor appears squirming in its Incubation Pod, ready to eject itself from the Citadel. Alyx taps the glass and this apparently angers it, sending telekinetic waves through the room as it ejects from the Citadel at top speed.
Throughout the rest of the game, an Advisor can be briefly seen on cameras giving orders to Combine units.
After the powerful Strider is brought down at the end, Gordon and Alyx escape City 17 via train as the Citadel explodes, creating a super portal through which more forces may arrive through. The last shot of the game is that of the Advisor pods dominating the skies as the Citadel explodes, sending the train off tracks.
Under orders from the Combine Empire, The Advisors are once again the main antagonists, taking a far more direct role and even engaging the protagonists. The most prominent among the Advisors is the Barn Advisor (the one who literally crashed in a Barn safe house).
The Advisors (known as Shu'ulathoi by the Vortigaunts) are still in their Incubation Pods and have not yet hatched, but are safeguarded by the most powerful Combine forces on their journey to destroy White Forest.
The Barn Advisor, however, ejected itself to safety from the Citadel during the events of Episode 1 and crashed in a nearby barn, emerging from its pod and building a protective life support structure around itself to rest and mature in growth and abilities. Gordon and Alyx encounter the crashed pod and begin experiencing powerful illusions and telekinetic disturbances as they get nearer to the Advisor. Alyx and Gordon decide to kill it before it wakes up by shutting off its life support system.
Gordon does so using the Gravity Gun, however all this does is cause the structure to peel back and reveal the now-awakened Barn Advisor, who casually slams Gordon and Alyx up against the wall and begins testing its powers on the surroundings, picking up objects and draining a dead man of his blood before twisting him and effortlessly throwing him across the room. It then decides to feed on Gordon until the structure encasing it explodes, resulting in it being injured and enraged by a bar being stuck partially in the side of its head. Deciding to leave immediately and heal itself, it tears the roof to shreds and tosses Gordon and Alyx to the ground, using the pieces of the roof as a powerful makeshift shield swirling around it before flying away.
As Gordon battles the Combine army to protect the silo within White Forest, an Advisor hovers over the entrance before Gordon secures it shut, sending more troops through. It is unknown if this is the Barn Advisor having healed itself (notice there is no metal bar or any sign of injuries) or one of its accomplices having awoken as well.
After Gordon successfully defeats the army of Striders and Hunters attacking the White Forest Rocket, Gordon presses the button and the rocket destroys the super portal, preventing more Combine forces from coming through and foiling the Advisors' plans yet again. Now extremely enraged, the Advisors decide to deal with matters personally.
As Gordon, Alyx, and her father Eli begin preparations for the former two to take off using the helicopter to find the Aurora Borealis (a ship manufactured by Aperture Science which appears to be a weapon both the G-Man and the Advisors could potentially use against the other side) and destroy it to prevent it being used, a furious Advisor tears through the roof of the garage and floors the trio, pinning Gordon and Alyx against the wall and beginning to attack Eli. Eli reminds them to destroy the ship before fighting back against the Advisor with a pipe and whacking it in the face, only annoying it further. Before he can do anything else, the first Advisor smacks the pipe out of his hands and a second Advisor arrives, picking up Eli as the first grabs Gordon and Alyx. Eli tells Alyx he loves her and to turn away and don't look, knowing exactly what the Advisor will do to him.
In mere moments, Eli has his brain pierced by the second Advisor's tongue and has the information in his mind drained as well as his life energy. With Eli finished, the first Advisor personally prepares to do the same to Gordon, however, help is on the way as D0G tears through the roof and heavily injures the first Advisor, causing it to drop Gordon and Alyx and fly out the roof to recover. The second Advisor promptly drops Eli and follows suit, both barely escaping with their lives but causing a major blow to the hearts of the Resistance.
Much of the Vimans over view about them due to the fact that all of them are now transraced by the Combine.
The Advisors are large, pale, grub-like creatures with no discernible facial features, save a gas mask-like device attached to their front end as well as a cybernetic eyepiece on the left-hand side of the faceplate. Advisors possess a long, prehensile proboscis, which they use to feed. They do this to humans by plunging the proboscis deep into the back of the neck. Exactly what they feed on from humans is unknown, though given the position it is most likely blood or spinal fluid. Each Advisor wears a skintight olive-green body suit that covers all but both ends of their bodies, with a collar around their "necks" adorned with golden glyphs. Some Advisors also have a pair of spindly black robotic arms strapped to their back that are capable of grabbing and lifting an adult human. During the Golmar Earth Uprising, Advisors are seen grabbing humans, both living and dead, and plunging the proboscis deep into the back of their necks. Combine Advisors have a wide range of odd vocalizations, from robotic growls and groans, to loud shouting noises and screeching. Besides their technological prowess, the Advisors also possess incredible psychic powers. At somepoint after they were completely Trans, the Emperor of the Combine Race destined to have the them majority into Advisors of the Empire which is mostly unknown what they do as Advisors, most likely they council with the Pure Combine Government's officials and Nobility (much like what The Old One had when he was an Advisor to the Emperor) also they had advised Occupied Planets that were ruled by the Combine Empire.
Advisors possess considerable telepathic and telekinetic abilities. They are able to communicate telepathically with Dr. Breen , as demonstrated in the Golmar Earth Uprising , and launch psychic attacks to ward off unwanted attention. Such attacks take the form of mostly harmless visions which warp the perceptions of those affected. They also evidently cause severe headaches. The Old One once said that they still had telekinetic abilities both pre and after their conquest.
Advisors also possess telekinetic abilities, allowing them to manipulate multiple objects with considerable force. They can completely immobilize several adult humans, crush a metal barrel, fracture a human skeleton with ease, and pull wooden planks off of buildings to use as protection. They presumably also levitate themselves with telekinesis. When they take to the air, loose dust and debris can be seen whirling around them, possibly as a side-effect of the force that levitates them, though this was only added as a method to prevent the player from firing upon them. Though they are able to concentrate on multiple independent objects, they cannot maintain their abilities under immense pain.