From a lore standpoint? By comparison how powerful would he be if his power wasn't being so heavily regulated and sealed? Just curious of how strong a being (once more, lore wise) he is compared to other of the more dangerous members of the league.
Well in the original lore it was implied that he'd be pretty much omnipotent once he gets out of that sacrophagus(or at least so powerful that nothing could stop him).
Now after the retcon it isn't that sure how powerful he'd be/is.
We know that 
&
sealed him away in the sacrophagus,but Renekton decided to be sealed away with xerath instead of fighting further against him,which implies that the 2 ascended weren't able to kill Xerath,even if they fought together.
Xerath was then trapped with Renekton for possibly centuries and it drove Renekton mad(though Xeraths mental health probably suffered as well)
A good deal more powerful than either Nasus on Renekton at least, as the two brothers together were only able to trap him....and one had to sacrifice himself to do it.
As far as how powerful he is in regards to the rest of the cast...depends on how powerful you view the big potential heavy hitters. Syndra is powerful enough to casually keep her flying castle going Brand is a man possessed by a fiery spirit of destruction Anivia is a constantly reviving Ice deity Until a potential lore update Kayle and Morgana are from another plane of existence. There are the otherworldly creatures of the Void
and of course Jax if he had a real weapon....
One could say he is the energies of ascension made manifest. Nasus, Renekton and Azir have all been altered but I'd say unless someone is a skilled sorcerer then one would be obliterated by the overload as seen with Azir in his and Xerath's backstory.
Bard is the only deity of Runeterra that we know of at the moment, so we can assume it is the most powerful.
Nasus, Renekton, Xerath, and Azir are all ascended beings, making them probably the next tier of power. Kalista is very nearly an avatar of vengeance, so I'm assuming she's like a minor goddess. She probably rests around here in comparison as well. Unbound Xerath is likely just as powerful as the others, as they all underwent the same enchantment. Why Xerath is roughly equal to Azir in power in League can only be described as a balance decision.
Most of the other magical champions fall beneath them, but their powers are often too useless or too hyperspecific to effectively categorize. Some like Lissandra, are very close to Azir; however, Lissandra is implied to have lost Shurima to the desert in one of her monologues, indicating that her power is less than Azir's.
At the bottom are all the champions whose combat strategies revolve around whacking each other with various objects. These are just random beings who are capable of holding objects. I can do that.
He is magic itself.
It can be trapped, it can be contain, but it is limitless. No matter what you do to trap him, he can simply wait it out.
No matter how strong someone is, their bodies can die while his cannot.
I still think Xerath is the most powerful champion in lol as lore-wise. Even more powerful than Bard.
He got prisoned yes, but I connect it that because of he was so naive to use his powers and it was his weak moment. Ascended brothers used this time perfectly. But even when he got prisoned they couldn't defeat him, tried to prison even more.
Also it's confirmed Bard is not a deity, god, omnipotent. Don't need to say it thousand times more. Bard can be destroyed by powerful spells or can be trapped like Xerath.
If there was a way to destroy Xerath I think brothers would do it. No one can know how to destroy an ascended better than them. They're from the same kind, they're the oldest ascendeds as we know, older than Xerath and Azir. Even when they combined their power and trapper him, they weren't near to defeat him, how could they kill?
