People must learn to hate. I will be their teacher.
Most villains have a quality that makes the reader like them. Some are their humour, with the likes of Loki, while some are liked for their cool design and demeanour, like Magneto or Doctor Doom. Some for their clever wit (Lex Luthor, The Shade) and others for simply being badass, like Thanos.
Cassandra has none of those. She's a special kind of villain that is utterly grotesque in everything she does, and isn't painted as someone who you love to hate, like most villains. She's written as simply being a horrible person with nothing positive about her, and everything she does isn't played for laughs.
Nova was born as the twin sister of Charles Xavier, and was killed while in the womb with her twin (long story, there was a pretty brutal fight) and she spent the last 40 years thinking and rebuilding her body from scratch. Since then, she's done nothing but try and kill as many mutants as possible in the worst of ways, regardless of context. She eventually ended up killing millions of mutants in a mass genocide, annihilating the entire nation of Genosha.
Even after that, she kept going. Basically the dark reflection of all of Xavier's values.
She's able to easily tangle with Omega level mutants with ease, and is physically immortal, as her Mummudrai self is nothing more than a spirit and thus has to be trapped or contained somewhere in order to stop her.
Feats here will be simple- Telepathy being the main category.
Blocks the city of Los Angeles using her TP and keeps Psylocke from bypassing that field. (Uncanny X-Force #13)
Uses TP to turn Beast feral. Nova did this while also using her TP to hide herself from the telepaths in and around the mansion at the time. (New X-Men #117)
Uses TP to control Rachel Gray from countries away. (X-Men Red #8)
Uses TP to become invisible from other people. (X-Men Red #3)
Can semi block Spiral's teleportation. (Uncanny X-Force #15)
Can use the bodies of people who died and uses multiple of them freely. (E Is For Extinction #4)
Casually TP's Lilandra into trying to die. (New X-Men #125)
Controls a bunch of people while fighting other people using her TP. (New X-Men #126)
Controls and makes a person's head blow up from range. (X-Men-Red #1)
Controls Forge using telepathy. (X-Men Red #6)
Beats Gladator just by taunting him and taking advantage of his mental state This isn't even telepathy, I just fought it was cool. (New X-Men #126)
Emma can't get through her strange psychic defences (X-Men #121)
Knocks out Spiral using TP. (Uncanny X-Force #13)
Slightly weakened Jean and Charles can't get a hold on her mind, knocks them back with a telepathic blast. (New X-Men #116)
Manages to bypass Jean's defences while she's distracted, showing her skill in finding telepathic weaknesses in people. (X-Men Red #8)
Nova boasts about Emma not having a chance against her. (Astonishing X-Men #13)
Controls all of Genosha (X-Men Red #10)
Possessed Bishop and made him time travel to the past. (Uncanny X-Force #5)
Stomps Xavier in TP so hard he can't even get a thought in. This was while he was using a device that amplified his TP. (New X-Men #114)
Telepathically takes out Cyclops. (New X-Men #126)
Takes control of Phoenix Jean Gray while Xavier was trying to fight Nova telepathically. (E Is For Extinction #4)
Manages to slowly manipulate Emma Frost with a tiny piece of herself while the latter was transferring Nova's mind. (Astonishing X-Men #18)
Can fly pretty fast using TK. (X-Men Red #11).
Annihilates a hallway with TK. (X-Men Red #11)
Cuts Logan's arm down to the metal. (New X-Men #116)
Twists someone's neck using TK. (X-Men Red #3)
Summons a giant rock elemental. (Uncanny X-Force #14)
TK's Psylocke (Uncanny X-Force #13).
Uses TK swords to beat Bishop and Storm before they can do anything. (Uncanny X-Force #15)
Can consciously copy people's DNA over a long period of time, as well as phase through people to kill them. (New X-Men #115)
Cyclops's blast does nothing to her body. (New X-Men #116)
Databook explains her DNA abilities, and how she can use them. (X-Men- Messiah Complex Mutant Files)
Genetically modifies her body to die quicker in anticipation for Xavier taking control of her body (New X-Men #122)
Cyclops's optic blast does nothing. (New X-Men #125)
Has a passive healing ability that allows her to heal from injury pretty fast. (New X-Men #116)
Phases through Wolverine's claws and a wall. (New X-Men #116)
Heals from having her neck snapped in seconds. (New X-Men #116)
Nanite is found in the brain and is very hard to take out. (X-Men Red #4)
Sentinels are able to take over people's brains, making normal people to unrelenting mutant hunters. If a mutant has one, they may try to kill themselves as well as whoever is around them. ( X-Men Red #4)
The Nano Sentinels are able to fly around and easily enter people's bodies and control them in seconds. ( X-Men Red #4)
With Forge, produces Nano Sentinel swarms that are large enough to infect entire cities. (X-Men Red #8)
She's decent, lol
Seriously, her TP and other abilities easily make her a high tier threat and beyond. In terms of pure mental strength, she has very few equals, and even less dudes who could be considered superior to her. Very dangerous, and a focused threat against anyone who she just doesn't like.