Tanith Low is a tritagonist turned antagonist in the Skulduggery Pleasant series. She was originally one of the main protagonists until she was possessed by a Remnant and transformed in process. By the end of the The Dying of the Light, she eventually returned of her normal self after Remnant that supposedly permanent inside her violently extracted by Darquesse, which also erase every single memories that she had while became evil due to the said remnant's possession.
Last Stand of the Dead Man
Tanith returns disguised as a rare female Cleaver tasked with relocating Dexter Vex. Vex regards how uncommon female Cleavers are, at which point Tanith reveals herself and kills the two other Cleavers, before kidnapping Vex. Tanith and Billy-Ray later hand over Dexter to Skulduggery, Ghastly and Valkyrie and even want to join the Irish Side. After a brief scuffle between Ghastly and Sanguine, the three agree and send Tanith and Sanguine with Donegan and Gracious to disable the Midnight Hotel which is the only way the Supreme Council can send their troops in through the shield. The pair succeed in their mission, but Billy-Ray and Tanith are captured. They are later freed by Aurora Jane and they all escape. Tanith is later captured alongside Valkyrie and Gracious. Using Darquesse, Valkyrie frees herself and the threesome escape. Tanith then travels to the English Sanctuary with the Dead Men and Valkyrie to get the Engineer's memory, and the mission is successful. Later on, Tanith attempts to kill Stephanie, and nearly succeeded, if not for Skulduggery, who intercepted her.
The Dying of the Light
Tanith returned into hiding with Billy-Ray Sanguine in a safehouse in Ireland, protecting Darquesse as she 'hibernates' as she requested. Originally overjoyed at protecting her messiah, soon Tanith began to lose faith in Darquesse.
When fighting their way to the Receptacle, Tanith was disappointed at the fact that instead of easily killing the Cleavers, Darquesse decided to fight them face-to-face, which did not sit comfortably with Tanith since she thought it was a waste of time.
When Tanith learnt that Darquesse has been separated from Valkyrie Cain, she attacked Valkyrie and her reflection in their home at Haggard and demanded for her to bring Darquesse back. Valkyrie explained that Darquesse was now a separate entity. Tanith admitted that her human side was beginning to question why she would want Darquesse to burn the world, and even her Remnant was beginning to agree.
Tanith helped Valkyrie escape from Darquesse in the caves under Gordon Edgley's home and cut off Mercy's head.
Darquesse questioned Tanith and then pulled the Remnant from her body to gain the knowledge of Kenspeckle Grouse, nearly killing Tanith. Darquesse healed Tanith because she didn't really want to hurt her. Tanith woke to see Billy-Ray and couldn't remember her time as a Remnant.
Because of Remnant's possession, Tanith drastically turned into the malevolent killer whom devoted to ensuring that Darquesse rises to power. She is shown to have second to no morals, killing people for money, recreation or even because they got in her way.