The Lore
https://universe.leagueoflegends.com/en_US/story/champion/pantheon/
As the enemy marauders closed in, an unnatural, keening wind descended, whipping the snow around in a growing fury. The swirling clouds parted, exposing the full majesty of Mount Targon at the epicenter of the storm. Warriors on both sides struggled to maintain their footing, shielding their eyes against the ice storm as a ghostly, glowing city appeared in the heavens at the mountain's apex.
The four stars of the Pantheon constellation pulsed brightly, then turned dark overhead. Simultaneously, the burning light of a falling star appeared within the ethereal city and streaked toward the ground.
It screamed toward the temple, moving at astonishing speed, and the barbarians prayed to their heathen gods in quivering voices. The streaming light slammed down, striking the ground between the two forces with an earth-shattering impact.
This was no star, but a warrior mantled in starlight and bearing a gleaming, golden shield and spear of legend. He had landed in a warrior's crouch, one knee lowered to the ground, and as he looked up at the enemy defiling the lands of Mount Targon, the Rakkorans saw it was Atreus... and yet not Atreus. The Aspect of the Warrior had infused him, and he was now both mortal and immortal, the incarnation of war made flesh. He was now an avatar of battle. He had become the Pantheon.
The Feat
Targon is a good bit higher than the clouds. We'll use Stratus clouds here.
Stratus clouds form at ~2000 meters
2000 / 41 = 48.78 m/px
48.78 x 579 = 28243.902439 meters
Low-End:
5 Seconds
5648.7804878 / 340.46 = Mach 16
Mid-End:
2 Seconds
14121.9512195 / 340.46 = Mach 41
High-End:
Done Instantly
28243.902439 / 340.46 = Mach 82
Using Grand Skyfall:
https://leagueoflegends.fandom.com/wiki/Pantheon/Abilities
Grand Skyfall takes a total of three seconds to execute, 1.5 seconds to reach the peak, 1.5 to come back down.
28243.902439 / 1.5 = 18829.2682927 m/s
18829.2682927 / 340.46 = Mach 55
Assalt's recalc
Since the timeframe isnt directly stated, a 15 second lowball is safe
28243.902439 / 15 = 1882.92682927 m/s = Mach 5.5,Hypersonic