Darth Nul, born Raniah, was a female Sith Lord of the reconstituted Sith Empire whose records were erased from history by the time of the third Galactic War. She was the co-creator of the Children of the Emperor alongside the Sith Emperor.
The individual who became Nul was born centuries before the Great Galactic War as "Raniah", a Jedi Master. Despite her rank, she roused uneasiness from other Jedi mainly due to her beliefs including her belief that anyone who could feel the Force deserved to be trained in its use.
Raniah was able to locate other Force-sensitives using an unknown technique involving mental connections which not only earned the praise of several Jedi Knights, Padawans, and even a member of the Jedi Council but she also earned suspicion from the remaining Jedi which prompted the Jedi Council to expel Raniah and her followers with the purpose of not allowing Raniah's dangerous ideas to continue.
With the Jedi no longer needing Raniah's ability to forge mental bonds, she later mistakenly made contact with the Sith Emperor Vitiate through the Force which she found that she could not withstand the Emperor's formidable power to dominate minds and ended with Raniah and her followers being pursued, captured, and enslaved.
Raniah, who was now under the Emperor's control, became the Sith Lord Darth Nul, and was responsible for some of the most treacherous mind-control techniques used to subjugate and punish enemies of the Sith.
Darth Nul had a temple on the remote Outer Rim planet of Elom. At some point, a relic created by Darth Nul was recovered by the Jedi Order and stored in a vault inside the Jedi Enclave on Dantooine. Darth Malgus had theorized that with Darth Nul's cunning, she must have had more than one location to house her inventions, aside from the Temple on Elom.
Darth Nul assisted the Emperor in creating the first Children of the Emperor, where she was tasked with locating the subjects and indoctrinating them to unconditionally serve him. Nul's inventions and designs were key to creating a Child and in conjunction with the Emperor's power she succeeded, therefore dubbing her as "the Mother" of the Children.
After an uncertain amount of time, Darth Nul's usefulness to the Emperor came at an end and he purged her existence from the Sith Empire's records.
Temple of Nul on Elom
Sith Lord Lana Beniko theorized that Nul's erasure from history had been done intentionally by either the Dark Council or by the Emperor himself with the latter case being confirmed.
The Emperor was able to create more Children, who he used to infiltrate the Galactic Republic and the Jedi Order, acting as his eyes and ears within the enemy's camp for decades, with some attaining prominent ranks within the Republic Military and the Jedi Council. During the Galactic War, the Children were exposed to the galaxy with the defeat of the First Son and were left to fend for themselves, with some renouncing their ties to both the Emperor and the Sith.
During the third Galactic War, through unknown means, the rogue Sith Lord Darth Malgus managed to learn not only about the existence of Darth Nul but her role as "the Mother" of the Children, as well as her unique inventions which Malgus sought for his own plans. Malgus began his plans with looting Nul's relic from the Jedi Enclave on Dantooine before traveling to Nul's Temple on Elom to locate a holocron which contained locations of "unworthy" Force-sensitive children. Malgus was subsequently defeated, but he secretly left the holocron to be found by the Jedi Padawan Sa'har Kateen.
After Malgus' capture, Talos Drellik was recruited to study Nul's temples and design due to his extensive career in Sith lore. After performing an extensive study, Talos found that Nul's devices including the "Regnant" device and the various traps the strike team encountered bear a striking, yet intriguing, similarities to the Phobis devices which were used to create the Dread Masters and the Ravager, a device used by the Sith Emperor to painfully extract secrets from his victims mind.