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