The Black Legions represented a highly skilled military force within the ranks of the Dark Jedi exiles during the era of the Hundred-Year Darkness. XoXaan, holding the title of Marchioness, commanded this unit. Notably, the Black Legions were involved in the conflict known as the Battle of Corbos, which took place in the year 6900 BBY. Furthermore, Sorzus Syn made reference to these legions in her own chronicle, which bears her name.
The initial mention of the Black Legions can be found within the sourcebook titled Book of Sith: Secrets from the Dark Side, authored by Daniel Wallace.