In the path towards the elevator shortcut (going left into the forest), look for a hilly spot with a giant fell tree. The Garb, Gloves, and Trousers are there.
The Mask is found by finding a spot where players can drop down into the path with the Celestial Minions, however, instead of dropping down, look for a spot that allows to jump across it to the other side. The mask will be there.
Though it leaves much to be desired on its defenses, it does have high all around Resistances and Beasthood, something that is almost unique to this set.
This set is appears to be a reference to the Crimson Set from Dark Souls. Like the Crimson Set it has a distinctive crimson color, all pieces of the set greatly resemble it.
The owner of this Attire, as mentioned in the description, is Dores. He was one of the two loyal servants of Master Willem, the other being the Gatekeeper that asks for the password in order to enter the Forbidden Woods. It is specifically mentioned that they became servants after being sent into the Labyrinths of the Chalice Dungeons, and turning mad after coming into contact with "the Eldritch Truth". As such, both Dores and the Gatekeeper can be confirmed as Tomb Prospectors, which are hunters that explore the Labyrinths.
The robe has multiple tools on the pack and the front. Though some are mentioned to be ritual tools, some also seem to be tools that fix mechanical parts.
The Graveguard Mask, bizarrely enough, appears to be the skinned face of a Pthumerian. Since Dores was someone who went to the Labyrinths of the Chalice Dungeons to perform rituals, it would seem that to have such a mask would perhaps make him go unnoticed.