AncientPower
The Chosen One
Darth Nox has debilitating and physically corrupting spells she can use on him to temporarily cripple him. She also has advanced stealth and teleportstion feats. She can heal herself or drain Marek at her own will. Her Force defenses have been strong enough to near effortlessly ward off the attacks of very strong Sith spirits and the likes of Zash and Thanaton.
Darth Nox a long time before her prime has killed Darth Zash with 'ease' despite Zash having been protected by Force resistant rituals.
Darth Nox has dominated the minds of a group of highly trained Sith assassins with apparent ease, demonstrating her 'madness' discipline mastery.
Nox 'destroyed' the 'supremely powerful' Darth Thanaton. Noting of course that Thanaton as Teneb Kel was so powerful that the Emperor personally appointed him to kill Exal Kressh.
Teneb Kel was strong enough that he killed a Gargantuan with Force Lightning despite being near death. He also threw around car sized pieces of starship junk with ease and formed a massive bridge of junk metal with tk in the vacuum of space, maintained that tk and withstood the vacuum of space as he moved across it. Teneb Kel was very powerful, decades before his prime as Darth Thanaton.
As Darth Thanaton he could kill Nox and Ashara Zavros with one Sith spell, all that saved Nox were the ghosts themselves. He was capable of withstanding the massive energy explosion that Nox unleashed. He later summoned a Bubble too powerful to be broken into. He has also of course summoned an FLS strong enough to rip chunks out of the Dark Council chamber and make Nox kneel. Nox again overcomes this with her spirits and proceeds to dominate him in return.
Darth Thanaton is not some peon, he's clearly very powerful and has feats to back them up. Darth Nox 'destroyed' him with the 'overwhelming power' of her spirits. A very impressive feat being desperately marginalised.