Alien X is an alien from the Forge of Creation. It is the host body to the Celestialsapiens Serena and Bellicus, Alien X's dual personalities: Serena, the voice of love and compassion and Bellicus, the voice of Rage and Aggression. For millions of years, none of them could agree on anything. And then, Ben 10 appeared, becoming their voice of reason, and finally Alien X could do things.
Alien X is omnipotent, with reality bending powers on a multiversal scale, able to affect space and time as he wishes. He could do anything he wants, right down to creating a near exact copy of the universe. This happened after the universe was destroyed by a doomsday device of mass destruction, called Annihilaarg.
Powers and Abilities
Omnipotent Existence/Reality Manipulation: Alien X is millions of years old. As Forge of Creation lies outside of the universal time and space, he could have existed since before the dawn of the universe itself. Or created sometime after the birth of it. Their knowledge is vast. And they are extremely powerful, able to affect the universe in any way they desire. Alien X had enough knowledge to recreate the entire universe to near perfect what it was with just a thought.
Weakness: As strong as they are, their weakness is the dual personalities. They could be stuck in argument for centuries on end. The only thing the Celestialsapiens, Alien X included agrees on is that Paradox should not come near them.
The other weakness they have is that their omnipotent powers can be absorbed. The entire quest of Agreggor was for the limitless powers of a Celestialsapien and almost succeeded in doing it. But because Alien X's powers come with dual personalities, anyone absorbing the powers will be riddled with the same dual personality syndrome. How agreggor sought to get by this was he was going to absorb a baby Celestialsapien that hadn't developed dual personalities yet.
In Alien Force, Ben refused to transform into Alien X fearing that Darkstar might absorb its power and become omnipotent.
Also Celestialsapiens are the reason behind the different art styles and appearances of the franchise.