I think someone answered this earlier, but basically she was from right after the first major war with the Sith, and was trained by a cabal of extremist Jedi who were trying to make completely sure that the Sith wouldn't rise again. She even had her name stricken from the roll of the Order so she could be used as a black ops person for the cabal.
The group figured out where the Muur Talisman was and sent her to get it so they could destroy or at least contain it. At the same time, she was supposed to assassinate an exiled padawan they had framed (obviously, they didn't tell her he was innocent of the murders he was accused of). She found both on Taris, but after meeting him started doubting that the padawan was guilty and so decided to prioritize the talisman. The padawan, unaware that she was supposed to kill him, decided to follow her and help with getting the talisman.
They tracked it to the ice planet Jebble where a bunch of Mandilorians were researching it. The talisman had apparently fused with the lead researcher and was kind of possessing him (that's just how the talisman works, it possesses people, but likes force users in particular). When the padawan found it, the talisman tried to jump to him, but Celeste saved him by letting it fuse with her, the more powerful force user, hoping she could contain it.
She only partially succeeded. She didn't go entirely crazy, but did start turning the Mandilorians into an army of rakghouls. By this point she'd decided there was no way the padawan was actually guilty and had kind of befriended him. He convinced her to be put in this stasis chamber specifically made to contain the Muur Talisman and its wearer, promising that it will only be temporary and he'd be there when she wakes up.
Unfortunately, in response to the mandilorian unit being turned into rakghouls, another mandilorian unit (led by the ancestor of Jango, I believe) nuked the planet while the padawan was off trying to communicate with someone to transport them away. He survived, but there was clearly no way she did so he left.
Except she didn't die and has been in stasis for nearly four thousand years. In the Legends timeline she's woken up by Vader and, appalled that he Jedi have been eradicated by the Sith (the exact thing her cabal was supposed to prevent), she duels him, loses, and then, to keep the Talisman from killing her and moving to Vader, she turns most of the rest of the people there into rakghouls and forces Vader to flee, stranding her on a moon empty except for her minions for decades, which drives her kind of insane.
There's a bunch more stuff about her fleeing to the unknown reaches after Luke finds her to keep the rest of the galaxy safe from the talisman, and then coming back and helping his decedent Cade Skywalker and having him destroy the talisman and kill her in the process. Basically everything in her life past her being frozen is horrible.
That's the not-exactly short but mostly complete explanation. She's part of an anti-Sith extremist Jedi organization, and Ciaran is - as Vectivus brings up in this chapter - technically a legitimate member of the Baneite Sith. At the same time, though, the Abyss Watchers as an organization are basically a more extensive version of what her cabal was trying to do. And her final mission probably very heavily changed her perspective on her bosses and force-sensitive secret organizations, but she never really got to display that character development in her original story. So there's like a fifty/fifty chance she either joins us immediately or tries to murder us.