Die....die.....die...nobody leaves here alive...
The ghost in the mansion is that of the last Himuro master, the 15th generation to have performed the ritual. The Mask of Reflection is supposed to change depending on the nature of the wearer, and it turned into the face of a Hannya demon during the last Ritual. When the Ritual failed, Malice drove the Master went mad and he went on a killing spree, decapitating the four priests and everyone he could find in the mansion. He finally committed suicide in the Great Hall with his long sword.
Even though the Mask of Reflection turned into the face of a demon, I'm not entirely sure this means that the last master was evil and malicious. The ritual was doomed to failure from the start, plus it's a pretty gruesome set of things he had to do to complete the ceremony, so...who knows?
The Master was responsible for keeping the secrets of the ritual and ensuring its success, so we know that he had Kirie's love killed to keep Kirie from developing attachments to the world. He also ordered the deaths of some other servants, since as Kirie's maid and the people who disposed of Kirie's love at his orders. My only explanation for why the Master demanded heads after he lost his mind is that he's probably the one who turned the crank that pulled the Rope Miko's head off, and when that failed...
On the Third Night, the Master can be found trailing blood through the mansion from the Moon Well to his final resting place in the Great Hall. His ghost is still insane; all it seeks is more heads to add to its collection.
During the Forbidden Rituals, four priests are responsible for aiding the Himuro master. They restrain the girl during the Blind Demon ceremony, and turn the cranks pulling the Rope Miko's limbs off. They each have a seal which is necessary to help unlock the Moon Well.
At the time of the last Ritual, at least one of the priests evidently felt sorry for Kirie's wretched fate, especially after her love was killed. The rest, however, seem to have stayed firm to their conviction that the ritual must be done to keep the Calamity from occuring. During the final failed ritual, the Himuro Master went mad and attacked everyone in the house, decapitating them. After the chaos settled down, presumably, the priests were buried with their seals under stone cairns around the Himuro grounds. They can be found in the Cherry Atrium, the Abyss, the Square Garden, and in the bamboo forest in the backyard. When Miku attempts to retrieve their seals on the third night, they emerge and attack with their spiritual power, trying to get their severed heads back.
But when they're finally exorcised, they express their misgivings about the last Ritual - "Kirie, poor girl...her only purpose in life is to die...if the Calamity comes, we're ALL dead..."
* back*