The Black Ajah Hunters were a group of independent Sitters of five Ajahs in the Hall of the Tower under Elaida a'Roihan during the White Tower Schism. They came together more by happenstance than by actual teamwork, partially due to the animosity between the Ajahs, but teamed up in a joint effort to hunt down the Black Ajah.
The Black Ajah Hunters were the following Sitters:
- Seaine Herimon of the White Ajah — Founder
- Pevara Tazanovni of the Red Ajah — Co-founder
- Saerin Asnobar of the Brown Ajah — Took command of the Hunters after having found them and become part of them
- Yukiri of the Gray Ajah — Joined along with Saerin
- Doesine Alwain of the Yellow Ajah — Joined along with Saerin
Upon discovering that her Keeper of the Chronicles, Alviarin Freidhen, was definitely chipping away her power and possibly plotting against her, Elaida do Avriny a'Roihan, a Red Sitter who had pulled down and stilled the former Amyrlin Seat, Siuan Sanche of the Blue Ajah, and taken her place as Amyrlin Seat, considered the time right to start looking for proof of Alviarin's treachery. She ordered a Sitter from the White Ajah, Seaine Herimon, to search for evidence of treachery "as high as needed be," hinting at the Keeper. Seaine assumed this meant to search for the Black Ajah.
To help her in this dangerous task, Seaine enlisted the help of her former friend and fellow Sitter, Pevara Tazanovni of the Red Ajah. It was not only her friendship with Pevara that made Seaine ask for her help; all of Tazanovni's family had been murdered by Darkfriends. Seaine therefore thought her suitable for the task at hand.
Experimenting and discovering
Together Pevara and Seaine worked out the theory that the Oath Rod can release a person from their Oaths, and while experimenting with their own Oaths, they were discovered by several fellow Sitters who had followed them into the Tower storerooms. Foremost of these was Saerin Asnobar of the Brown Ajah, not only the oldest, but also the one who had been a Sitter the longest time. The other Sitters were Yukiri of the Gray Ajah, Doesine Alwain of the Yellow Ajah, and Talene Minly of the Green Ajah. Upon explaining their purpose and orders from the Amyrlin Seat, Seaine and Pevara re-swore their Oaths and proved themselves not of the Black Ajah. Saerin, Yukiri, and Doesine also re-swore, but Talene refused, damning herself as Black Ajah. Talene was questioned on the Chair of Remorse, a ter'angreal constructed specifically for interrogating purposes, naming several other sisters as Black; most significant of these was doubtlessly Galina Casban, a leader among the Black Ajah and the Head of the Red Ajah, known as the Highest. The discovery that a fellow Sitter was Black was what cemented the group's cooperation; however, Saerin assumed command.
In near desperation on the hunt for other Black sisters, Seaine forced Zerah Dacan, a White sister of the Rebel spies, to swear on the Oath Rod, thereby betraying the other spies. Word of this, however, did not reach Elaida through the Hunters. Beonin Marinye, officially one of the instigators of the Salidar rebellion, was in fact Elaida's mole and had returned to the Tower, betraying the spies already. The Rebel spies joined the Hunt, but few of them contributed in an effective way.
Apart from discovering that Talene and Galina were Black, the group uncovered several other sisters named by Talene, such as Atuan Larisett, and through Atuan Karale Sanghir and Marris Thornhill. Temaile Kinderode was no real discovery, since she had already branded herself as Black by being part of Liandrin's group of Black sisters.
After the interrogation of Atuan, the group reached a problematic stalemate. Karale and Marris were both adamant that they walked in the Light again, but their Black contacts outside of their heart were among the Salidar Aes Sedai. Their hunt came to a halt.
Shortly before Pevara left for the Black Tower on orders from Tsutama Rath, the new Highest, Talene was called to a meeting with the Supreme Council, the thirteen highest ranking of the Black Ajah. Saerin had Talene hidden in the Tower cellars, but in the confusion following in the next few weeks, Talene fled the Tower. Pevara suggested that the Hunters, along with the rebel spies and the uncovered Blacks, attack the council and decapitate the Black Ajah in one strike. To this, Yukiri sarcastically stated that Pevara's shawl was "red, yes. I thought it had turned green when I wasn't looking." 
Meeting the Salidar Amyrlin
Meidani of the Gray Ajah, who was one of the rebel spies, was uncovered due to Zerah's unwilling treachery. Upon being ordered by Yukiri to renew the relationship she shared with Elaida during their time as novices, Meidani encountered Egwene al'Vere, the Rebel Amyrlin Seat who had been captured by the Tower loyalists. Egwene, noticing that Meidani was hiding something, made Meidani take her to those who had placed another Oath on her: the Black Ajah Hunters. Despite initial protests, particularly from Yukiri, the Hunters could not deny that Egwene was right in the wrongness of the fourth Oath, and Egwene earned Saerin's and Seaine's respect.
End of the Hunt
As Elaida was captured during the Battle of Tar Valon and made damane, the Sitters in the Hall - and the Ajah Heads behind them - could agree on no other candidate than Egwene for a successor. After having seen her dedication to the Tower and strength as Amyrlin, the Hunters became Egwene's strongest allies in the united Hall. Their Hunt ended as Egwene either executed or smoked the Black Ajah out. 
A fourth Oath
The Hunters decided to take the risk of breaking Tower law by forcing a fourth Oath on their Black captives and the Rebel spies, an Oath of obedience to the Hunters themselves not to betray them or their cause. The move was necessary to avoid being discovered by the Black Ajah, but it was a violent breaking of Tower law and was not much better than Compulsion in some ways.
"The Wheel of Time Companion" reveals that Egwene successfully convinced the Hunters to remove this oath of obedience from the rebel spies.
List of the so called "Ferrets"
List of the rebel spies that were forced to join the Black Ajah hunt (with their Ajah and strength).
- Annharid Matoun Yellow Ajah 17(5)
- Bernaile Gelbarn White Ajah 16(4)
- Cathal Devore Brown Ajah 17(5)
- Celestin Eguilera Yellow Ajah 19(7)
- Ester Stepashin Green Ajah 16(4)
- Haesel Lusara Brown Ajah 16(4)
- Jennet Cobb Gray Ajah 18(6)
- Meidani Eschede Gray Ajah 17(5)
- Mikio Vadere Green Ajah 18(6)
- Zerah Dacan White Ajah 19(7)