So the reason Alastor made a deal with another demon (probably Lilith) is because he wanted to be stronger so no one could control him. Now here’s my idea: Alastor and Vox fell in love, but during a fight, Alastor was hurt because he was paying attention to Vox. Either that or he realized that Vox was holding him back, so he went to Lilith asking that she take away his emotions and give him more power while Lilith gets his soul in return.
Bonus: If and when Alastor breaks out of the deal, he has all the emotions and feeling he had about Vox return to him and they get together.