The keeper of souls and the underworld.

Corrupted by the dark powers to create the undead of the valley.

His power is kept in the Stone of Life.

He is the power that keeps the souls of the Barovians trapped within Barovia. Strahd is his host and has used his powers to make the Heart of Sorrows.

The Master Dusk is dormant and half asleep, he is not aware of the destruction strahd has carried out in his name.

The Master Dusk is basically a nature based Death god. But remember, he isn't evil. Death is supposed to be a natural occurrence, meant to be accepted and not feared. Strahd and other undead are therefore his natural enemies, as their very existences are perversions of death.

The henge like circle of stones atop Yester Hill was once a sacred shrine dedicated to the Master Dusk. The shrine itself is completely surrounded by graves, after all. Even the Gulthias Tree, a tree of undeath and evil, is representative of her power.