It's not so easy; before the first WA of the power, it was a populist dictator who made an alliance with demons to become Queen Vrinn I of Negeemiliel.
She was killed during the war, and her generals couldn't keep the regency for her daughter, and the negeemi Republican institutions where reinstalled -largely by the senatorials-
General Dahl'Arak ., being a senatorial and with a bad personal experience, is no friend with demons, so while Negeiliel will be shortly contacted after the return of demons, but she delayed any answer, and it wouldn't be until much later in the war that Negeemiliel was "convinced" to enter again in the dark legion - for more of the quuen of Negeemiliel, see the wiki