D&D 5E (2024) Backgrounds in the Forgotten Realms Books

[Necropolitan]

While they were dedicated to the deity Mask for many years, sometime after the Spellplague they shifted their devotion to the goddess Shar."

I think that's when Shar put her master plan to plunge all of existence into oblivion into motion and absorbed/devoured Mask.

Mask was later reborn when his divinity was reunited in one person, so hopefully he's taken over the Shadowmasters again.