Next to nothing is known about Alioth. It claims to be the supreme time being of the time-span prior to Kang's (approximately 2000bc-6000ad), stretching back to the dawn of life on Earth. It claims to be the first being to free itself from the timestream, presumably meaning to travel into Limbo, or to another time period.
Kang has claimed that it has only been though his efforts that Alioth has not taken over all time periods. Kang somehow managed to form a barrier to prevent Alioth from entering his portion of Limbo. Kang was rendered comatose in a battle with the Avengers and Fantastic Four (I believe this was the Citizen Kang saga). With Kang's defeat, his lover, Ravonna, aka the Terminatrix, used an image inducer to assume his form and took over his rule of Chronopolis. She began to explore the realm of Limbo and upon encountering a barrier, she blasted a hole in it to learn what was on the other side. This allowed Alioth to escape. She fled before its power, but Alioth soon traveled to and attacked the city of Chronopolis.
The Terminatrix, acting on assistance from her alternate future self, Revelation, revived Kang and informed him of Alioth's attack. Alioth invaded the Chamber of the Council of Cross-Time Kangs, slaying all present. Kang attempted to stop him, but was forced to flee to avoid his own absorption. Kang, along with the Terminatrix and Revelation, convinced the Avengers of Alioth's threat to their reality, gave them a Chrono Key, and sent them to battle Alioth. None of the Avengers power presented any threat to Alioth, so Thor activated the Chrono Key, which then assumed its true form, that of Tempus.
Tempus, initially the size of a hand-held figurine, absorbed the energy of the dead Council members and rapidly grew to enormous proportions. Having prevented Alioth from absorbing their energy, Tempus then attacked Alioth directly. Tempus continued to grow, feeding off of Alioth's own temporal energies until it reached reality wide proportions, creating the time barrier between Alioth's realm and Kang's. This barrier spread across the timestream, and is the same barrier originally encountered by the Terminatrix.
"Later" after his return from his second stint as Rama-Tut, Kang decided to take action against the other chronal rulers. He manipulated events to set the various organizations against each other. As part of this, Kang weakned Tempus slightly, allowing Alioth to break through into the realm of the Time Variance Authority, throwing them into turmoil.