Necropolitan
Hero
I think that's when Shar put her master plan to plunge all of existence into oblivion into motion and absorbed/devoured Mask.While they were dedicated to the deity Mask for many years, sometime after the Spellplague they shifted their devotion to the goddess Shar."
Mask was later reborn when his divinity was reunited in one person, so hopefully he's taken over the Shadowmasters again.