Aja's Coven was a coven of twelve witches who were presumably led by Aja. They came to Mystic Falls on Abby Bennett Wilson's behalf in order to help cleanse her daughter, Bonnie Bennett, of her Expressionmagic. When the twelve witches linked themselves to Aja in order to give her enough power to cleanse Bonnie, they learned that Bonnie had been working with the immortalSilas, which led Aja to decide to kill her for the good of the world. Caroline Forbes, unable to watch as they attempt to kill her best friend, stabbed Aja and killed her, and because she was linked to the rest of her coven, the other eleven witches died with her.
Nothing is known about them except that Aja was formerly in contact with Abby Bennett until she was turned into a vampire. They still felt an obligation to help Bonnie end her addiction to Expression magic, though, and they all came together in order to help save her in Because the Night. They were ultimately killed by Caroline in an attempt to save Bonnie from being killed by Aja after she learned that Bonnie was being manipulated by Silas to complete the Expression Triangle. However, when Caroline ended up killing Aja, it resulted in the completion of the triangle, despite their attempts to prevent it from happening. Their deaths caused the spirits of the coven's members to feel so much animosity towards the Mystic Falls Gang that they joined forces with Kol Mikaelson and Tyler Lockwood's former hybrid pack in order to get their revenge on them after Bonnie had lowered the veil separating the living world from the Other Side in Graduation. The group intended to kill the gang in their corporeal ghost forms, but they were quickly and easily thwarted by Niklaus Mikaelson, who beheaded Aja with a graduation cap. The rest of the coven quickly scattered in fear at the sight of the Original hybrid and tried no further attacks on the living while the veil was down.
Aja's coven appeared to help Bonnie to get rid of her Expression magic.
They came back as ghosts to get their vengeance after being killed by Bonnie's friend.
It is possible that Abby Bennett was formerly a member of this coven prior to being turned into a vampire, because traditionally, witch covens are made up of thirteen members. However, this remains to be proven.