Before the events of the original Half Life, there were 3 "powers" that resided in this universe and vied for control over it. Now, when I use the word "power" I use it in terms of super-sentient beings, races, military, etc. It's a term to cover all of those. The main ones in this universe are as follows:
THE NIHILANTH AND THE VORTIGAUNTS
I include the Nihilanth with the Vortigaunts because after the events of Half-Life, the Vortigaunts are scattered, and therefore become a smaller power. They still function, however, without their leader. This can be an interesting plot point for Half Life 3 as well. Our theory is thus:The Nihilanth and the Vortigaunts were being chased from their home-world by The Combine, a military organization of the mysterious other power (Overlord...?) as they were losing the battle. So the Nihilanth lead his people to Xen, the gateway world, where I believe
G-Man used to reside, before the events of Half Life. Using the combined power of the Nihilanth and the Vortigaunt, they nearly destroyed the G-man, but him being what he is, he remained, but only in a smaller, weaker form. There, the Nihilanth sealed the opening from their homeworld, and barricaded The Combine out of Xen. There, they began to plot their next move. Meanwhile, the G-Man used what remained of his power to scout out around the universe and find "champions" for him to reclaim his home-world. (The Combine are the "champions" of the Overlord.) After the events of Half Life, the (spoiler alert) Nihilanth dies, by the hands of G-Man's champion, Gordon Freeman. Now, the Vortigaunts are scattered, but together some are strong enough to defeat a weakened G-man, because of their first battle with him.
THE G-MAN (AKA THE EMPLOYER)
The G-Man has always been that mysterious watcher and "employer" in all the Half Life games so far. He seems to have power in our universe, but for a supposed "God" his powers seem limited, if a few Vortigaunt can defeat him (In Episode 2 the Vortigaunt prevent him from taking Alyx). So our theory is this:
G-Man was the supersentient powerful being residing on Xen. Originally peaceful, wishing nothing to do with anyone, he sat there in solitude. Until The Nihilanth and his people arrived, hostilely took over, and nearly killed him. Then he became rather angry with them, and searched the universe for a new "champion". He found 3 champions, the day of the resonance cascade in Half Life, one being Adrian Sheppard, one being Eli Vance, and the other (temporarily) being Gordon Freeman. Adrian Sheppard is locked in stasis, after the events of Half Life: Opposing Force. Eli Vance escaped, and then became head of the rebellion, after refusing the G-man's offer. Kleiner would have done so as well, but he was not considered a viable asset to the G-man. He then proceeded to use Freeman to kill his enemy, the Nihilanth, and destroy the evidence using Sheppard. Then, he aided The Combine during, and a bit after, the 7 Hour War. As the old addage goes, the enemy of my enemy is my friend. But realizing his alliance with The Combine won't end well for him, he brings Freeman, his main champion, to take down The Combine. After fracturing the Combine army, he finds that Alyx Vance is just as powerful as Freeman, and tries to take her as his new champion. She was, however, a Vortigaunt champion. After a conflict with the Vortigaunt, however, he loses both Freeman and Alyx. Furious, he decides to kill Eli, to try and cripple Alyx's mental and emotional well-being, and he takes back Freeman as his champion again. Our evidence for this is in his speech to Adrian Sheppard, saying that "I have a fascination with those who adapt and survive against all odds. They rather remind me of myself." It would seem to prove our theory.
THE COMBINE & THEIR OVERLORD
Now this one is a bit trickier to explain. Our theory is that the Grand Overlord (or whatever) presides over all The Combine and The Overseers. He gives the orders, and as powers go, started as the weakest, and had the longest reign over his own domain. .jpg)
When his race and the Nihilanth went to war, he used his powerful Combine army to overwhelm Nihilanth's forces, and push him to Xen. After Nihilanth set up the barricade, the Combine waited patiently and amassed troops for a full-scale invasion. What The Overlord did not expect, however, was that he was not to take over Xen, but Earth. When the Nihilanth died, The Combine came through, expecting a massive fight, but all they found was the remnants of the Vortigaunts and the Nihilanth. Then, turning their gaze to Earth, easily took over. There, they enslaved the Vortigaunts below humans, and essentially, defeated them, with the help of the G-man. There, he reigned over both the Earth and the Combine Overworld. Then, G-man double-crossed him, and crushed his massive army, and defeated him at every turn. His last bastion is in the North, where they (most likely) discovered the Borealis, and used it's technology to call reinforcements.THE "CHAMPIONS" OF THE HALF-LIFE UNIVERSE
GORDON FREEMAN
As to be expected, Gordon Freeman, is the most powerful of all champions. He is somewhat like a "power" but he has no control over his abilities. Wherever he goes, he brings with him great change. He's a "demigod" if the "powers" were gods. As the Player Character, he must be though. Otherwise the story wouldn't be fun, and wouldn't be Half-Life.
ALYX VANCE
An equal to Freeman in power, but not nearly as developed. Her physical and mental ability make up for her lack of "magical" abilities. She shows this through kicking more ass than Freeman sometimes, and constructing D.O.G. This is something, as much as the MIT grad can be intelligent, he cannot do. Her close connection to her father, however, proved to be a weakness, after the events of Half-Life 2: Episode 2.
BARNEY CALHOUN
A smaller champion, but still slightly powerful, Barney was the Player Character in Half Life: Blue Shift. He is Gordon's best friend, as has always been a "man on the inside" providing intel and insight into the other side.ADRIAN SHEPPARD
A military personnel from the American government, sent to Black Mesa during the events of Half Life, to "cover up" in the incident, as well as eliminate Race X from Earth.and in doing so, he became a massive part of this war. G-man used him to get Race X off Xen, so he can reclaim his homeworld.WALLACE BREEN
Originally the head of Black Mesa, when the 7 Hour War began, he was the Ambassador for Earth during the surrender. Then, he was appointed to the Administrator of City 17, but basically the whole planet. He then became The Combine's "champion" and served them until his death at the end of Half Life 2.ELI VANCE
Originally a colleague of both Gordon and Kleiner, he was at the resonance cascade. He became the head of the rebellion against the Combine after the events of Half Life, and Eli Vance is the father of Alyx, and was originally aware of G-man, but wanted nothing to do with him. He then becomes a "champion" of the Vortigaunts. After the G-man gets duped by the Vortigaunts, G-man kills Eli to make up for it.