After the Beyonder and the Molecule Man learned that their power derived from the dimension that created Cosmic Cubes, they merged themselves together into a new Cube. This Cube, as with the other Cosmic Cubes, continued to develop its own powers and self-awareness. Eventually it sent the Molecule Man's self back to Earth, and it turned into a new female humanoid being, Kosmos. This was surprising, since the Beyonder had chosen to appear in a male form. She later explained to Kubik (another sentient Cube like herself) that she felt that taking on the female form would obviate some of her previous problems, which had been partly derived from her male format.
Kubik took upon himself the tutoring of the new being, who had forgotten much of what she learned in her previous life. In their first session, he took her on a tour of existence, showing her the various dimensions and cosmic entities in the multiverse, including the Phoenix; the Watchers; various combined life-forces such as Mangog, Overmind, and the Uni-Mind; the High Evolutionary; the Stranger; Eon; the Celestials; the Vishanti; the various forces of evil; Death; Master Order and Lord Chaos; the In-Betweener; the Living Tribunal; Galactus; Eternity; and the Beyonders (plural--the source of the Cosmic Cubes). Kubik appreciated and encouraged her questions about her place in the multiverse; it was unclear where exactly Kubik and Kosmos fit into this hierarchy of power.
Later on, Kubik and Kosmos became close. They continued to tour the multiverse together as Kubik taught Kosmos about existence. They were not above manipulation of beings that they saw as being "below" them: at one point the two of them created a moon-sized space station and ran the Fantastic Four through a series of exercises like lab rats. It was during one of these sessions that the Molecule Man, who had reverted to evil, pulled the old male Beyonder essence out of Kosmos, leaving her a pale unconscious shadow. The Molecule Man defeated the Beyonder, and Kubik showed up asking him to stop. Kubik said that he and Kosmos were pure power--that if their power was taken, they would be nothing. But the Molecule Man had greater power, because of his human potential. The Molecule Man was moved and restored Kosmos to her former state. He later said that even as he was a human who was learning to be omnipotent, they were omnipotents who were learning to be human.
The "Maker" Phase
Sometime later, due to unexplained events, Kosmos became mentally ill, and started calling herself the Maker. When she destroyed an entire Shi'ar settlement known as Eru 7, she caught the attention of the Imperial Guard. She did not put up a fight, and Oracle shut down her mind. She was found guilty and sentenced to the Kyln in Creche confinement.
Gladiator learned through Oracle, who had taken some of her madness, who the Maker really was. It seemed that Kosmos had tried to take an even more "mortal" form than before, perhaps somewhat similar to what the Beyonder was trying to do at the end of Secret Wars II. She was now bluish white, instead of gold. This extra degree of mortality appears to be what turned her insane. (It is unclear why Kubik did not help her.) Gladiator volunteered to be placed in the Kyln to keep an eye on her and destroy her if necessary. A riot at the Kyln set her free. She gathered a following of inmates who worshipped her. Gladiator and Starlord, among others, opposed the Maker and even gained the help of Thanos, who was visiting the Kyln on a pilgrimage.
When the Beyonder's personality resurfaced and drove her insane, Thanos was forced to shut her mind down and turn her into a living prison so that the Beyonder could never escape unless Kosmos was somehow killed. She was placed in stasis at the Kyln and kept alive so the Beyonder could not escape.
The Kyln prisons were destroyed during Annihilation, and the Maker was apparently killed in the battle; Thanos had the Fallen One check her body to make sure. (Since it was implied that her essence would escape if her body was killed, then it would seem that her essence should be on the loose, but this has never been dealt with or mentioned.)
Kosmos did not use her powers much. However, it can be inferred that her powers were more or less on the level of other sentient Cosmic Cubes like Kubik and the Shaper of Worlds, which are in essence limited omnipotents. In her battle with Molecule Man, the effects of their battle affected distant times, dimensions, and galaxies.
However, in her Maker persona, her powers were vastly more limited, perhaps due to her incarnation in a more mortal body. She lost a psychic battle with Thanos, although this was at least partly due to her naivete in the ways of battle. Still, Gladiator called her (perhaps with typical comics hyperbole) "the most powerful being in the known universe" even in this stage of her existence.