A few comments.
- He's a contract devil. He can't do much to you unless you (or someone else you know) accept his deal. This specific obsession binds him to certain rules and behaviors he will not willingly violate, and if you just don't associate with him you're probably perfectly safe.
- I'd say Kindred are two of the weaker parties in Runeterra simply because they're heavily bound by rules. They can't affect anyone who isn't supposed to die. Even if they could, a quarter of the roster has to be destroyed through violence because they won't ever die on their own, and there are a few champions that simply cannot be destroyed.
- What a big fall from grace for Nasus, who used to be in the Top Three. The Ascended are powerful, but their power seems to be finite and regional. Azir might have a great deal of power in Shurima, but I doubt he'd be so impressive outside of it simply because his powers rely a lot on sand. Nasus and Renekton seem to be weaker than Azir is. Overall, the Ascended aren't that impressive. Except Xerath.
- The most powerful Ascended and the only one that doesn't seem to have any real limits, either self-imposed or otherwise, on his powers beyond that sarcophagus Nasus threw him in. If he ever shakes off that sarcophagus, he's going to be able to do pretty much whatever he wants.
- Lissandra would be dangerous if she were intelligent, but luckily for the world she's so completely beholden to the Watchers that she seems incapable of acting on her own initiative. If she'd used those thousands of years after she got her revenge on (killed) Avarosa and Serylda, time which she was basically unopposed, to actually try taking over the world (or at least the Freljord) herself THEN bring the Watchers back she'd probably have succeeded. But no. Great powers; she'd be a credible threat if she weren't also an idiot.
- The fact the lore is split between old and new makes some champions impossible to gauge, and with these two not having had new lore in basically forever it hits them hardest, since in the old lore Kayle was easily in the Top Three (and it's implied Morgana can match her) but now we just don't know. If we still count the old lore, we know Kayle doesn't fight anywhere near full strength (She's been confirmed to use Intervention on six people at once, for instance) and Morgana is supposedly able to rival her (though her old lore doesn't say how). Both of them are powerful enough that the head of the League and its most powerful summoner specifically sought them out, and Kayle remained the League's spokesperson, judge, bodyguard, and referee until the Great Retcon.
- Both rising stars that should be watched out for, but even at full strength they'd go down rather quickly to...
- One of the most seriously underestimated people in Runeterra, and its only his insane focus on being a Saturday Morning Cartoon villain that stops him from being able to do some real damage. Sure he's ineffectual and useless, but bear in mind that we've never seen him cast a single spell. What we do know is he's one of the scariest, most well-educated sorcerers in Runeterra and he knows the secrets of dozens of wizards and sages. He can warp cosmic power to his own purpose, drop meteors from the sky, and twist peoples' magical abilities back on themselves. There are champions that would give him problems, but he acts as a gate for other spellcasters - They can't be the most powerful unless they can deal with him, and the only three whose powers would give them an edge would be Kayle, Morgana, and Lissandra.