Since a while ago I did a similar list for Ryze, I thought I could make one for one of the only mages that surpass him. So let's begin.
It surpasses that of Nasus and Renekton, 2 high-tier Ascended. Nasus's most powerfull attack, his volcanic fire, only managed to make Xerath shriek in pain, but did not do any lasting damage.
**Superhuman reaction speed**
Xerath was able to go toe-to-toe with Nasus in terms of in-battle speed and react to his blows. Their battle was described as a blur to normal humans.
Xerath is able to fly (pretty self explanatory).
**Superhuman Physical strenght**
Tho he has no use for it.
**_MAGICAL ABILITIES AND SPELLS_**
Even as a human, Xerath was the greatest archmage of Shurima at that moment, and if he was still human right now he would still rank high, only being surpassed by the best of the best mages like Ryze, Lissandra, Vladimir, Brand, Syndra.
**Lighting bolts from the sky**.
Xerath was able even as a human to summon magical lighting bolts with a storm to strike with great accuracy Azir's father's room.
**Arcane beams from the sky**
Aka his in-game W, tho this is possibly a variation of the previous spell.
Pretty self-explanatory, they were able to instakill Azir's father and his guards, as well as Azir himself while the latter was still human, and able to melt bones to ashes instantly. Keep in mind, those feats I just listed were while he was still a mortal. Right now, his arcane beams are powerfull enough to severly hurt Nasus.
Again self-explanatory. I'm 90% sure Xerath can do this in-lore since in a Riot official art Xerath was shooting a burst of arcane energy from his chest to Lucian while charging up his Q with both hands.
He used it on Nasus and was able to almost one-shot him with that, as well as daze him. His in-game E, he wasn't even trying to kill Nasus with that.
Was able to pick Nasus up with it and throw him around like a ragdoll, crashing large pillars of stone with him, and also Nasus's bones with the sheer force of the impact of his telekinetic throw.
His in-game ult, is a combination of cosmic-magic fire and own arcane magic. It hurted Nasus, who like all Ascended has a passive resistance to fire. It was described as being cold but burning just the same, and Xerath was able to crumble down entire buildings of Vekaura with it, turning rocks to lava instantly.
During the siege of Vekaura, Xerath was mindcontroling his entire army from Nerimazeth to attack Vekaura. The mindcontrolled soldiers had no fear because of that, not even Nasus being able to scare them, making them more effective in battle.
In Taliah's color story, Xerath uses a deadly magical sandstorm from very far away to make the Shurimans leave the capital. It doesn't quite work for him as Azir provides them shelter, but it's still a very impressive feat nontheless.
Xerath can trap someone with arcane fire chains in-place. The quote where he did that: The magus hurled chains of white fire into the crumbling walls and said, “But until then, I entomb you beneath the sands as you once trapped me.”
Xerath blazed like a newborn star and dragged his fiery chains inward. A thunderous rain of broken stones fell in an avalanche as murderous fire poured from the sky to fill Vekaura.
He was only interupted cause Taliah intervened.
So this was it. Leave your thoughts and feedback in the comments if you want, as I plan to do more of these.