Alioth is the first being who broke free from the constraints of time. His empire is larger than that of Kang himself, stretching billions of years to before the rise of man on Earth. His domain is approximately x2 to x3 as large as Kang's temporal empire. It is because of Alioth that Kang never attempted to expand his empire vertically into more distant time eras and therefore extends only as far back as 2000 BC.
Kang created a barrier preventing Alioth from entering his domain. After Kang's death, Ravonna the Terminatrix accidentally released Alioth. Who then threatened Kang's former domain and all other temporal dominions further up the timestream. Ravonna and Revelation were able to revive Kang who provided the Avengers with the Chrono Key to use against Alioth. Which was in reality Tempus who upon being released inside Alioth grew to match it in power, locking the two in eternal combat. Later Kang would as part of his machinations to keep his rival temporal empires occupied, weakened Tempus so that Alioth would break through, invading the dominion of the Congress of Realities.
Alioth is a trans-temporal entity, existing across divergent timelines as an enormous cloud destroying all it touches, causing massive temporal disturbance, and devastation across entire dimensions. It can extend pseudopodia to attack focal beings or locations and grow in size and mass by absorbing the bodies of time travelers into its mass. Once it occupies a time period, it spreads out to engulf the adjacent realities. Alioth also negates temporal energy, preventing travel through it to a time before it exists. Attempts to travel through Alioth usually result in absorption into its mass.