Here is the full list of all 15 The Watchers achievements.
Mask returned to the Altar.
Goblet returned to the Altar.
Ritual Book returned to the Altar.
Sword returned to the Altar.
Bowl returned to the Altar.
Returned a cursed belonging with zero deaths.
Skull returned to the Altar.
All roses found.
All cursed belongings returned to the Altar.
Telephone returned to the Altar.
Mirror returned to the Altar.
All cursed belongings returned with zero deaths.
Shoes returned to the Altar.
Robe returned to the Altar.
Died 100 times.