One of the most mysterious, and certainly unsettling factions in Demon's Souls is the Secret Society (秘匿者, "Concealers"). Who are they? What is their purpose? Where do they come from?
The members of the Secret Society can be identified by their wearing of the Rogue's Set (more accurately "Concealer's Set"), glossy black robes with gold threads; and given that the Rogue's Set is a female-exclusive armor, as well as multiple item descriptions specifically mentioning women in relation to them, we can infer that the Secret Society is composed entirely of women.
Extravagant clothes made of soft black cloth combined with gold thread.
Garb of the Concealers who hole up in closed rooms and believe in the Soul Arts. Their suspicious true state is mostly unknown.
We can find every single bit of items about them exclusively in Latria, and fitting with the country's signature weapon being rapiers, the signature weapon of these women is the Epee Rapier. There is no doubt that the Secret Society is hidden among Latria's nobility.
Ceremonial thrusting sword embellished with intricate gold design.
Weapon of the Concealers who hole up in closed rooms and believe in the Soul Arts. A strong fire magic-power has been added. This is one of the reasons why the Concealers are so terribly feared.
The motives and goals of the Secret Society is to continue practicing Soul Arts during the Dark Age, thus why they always do it in secret, and why the fire magic of their Epee Rapier makes them all the more feared: with magic forgotten by the masses after its ban, the people can't comprehend the fire nature of the weapon, and so they fear it even more. As well, consider that their "true state" are unknown for most of the world - much like Soul Arts themselves.
The women of higher hierarchy in this Secret Society wear a gold mask unique to them, Mephistopheles being a good example. Yet we find a corpse in a cell in the Prison of Hope carrying almost the whole Rogue's Set and the Parrying Dagger (another weapon heavily associated with them).
A dagger specialized for parrying. Customarily equipped in the left hand.
The weapon of the Concealers who hole up in closed rooms and believe in the Soul Arts. They prefer to use this special dagger rather than a shield.
(It is important to note that here, the Japanese script of the Parrying Dagger is specifically referring to "they" as female)
However, this corpse does not yield the Gold Mask. How is that the lower hierarchy easily gets caught and tortured to death, yet the upper hierarchy still (even though secretly) thrives with their meetings and misdeeds? The only possible answer is that they have infiltrated into the Queen's clan and gain favors from it, perhaps even from the Queen herself. And when one of their lower members is no longer desired, they expose her, leading to her getting caught, imprisoned and killed, successfully washing their hands. In fact, Mephistopheles drops a Ring of the Accursed upon death, yet she doesn't wear it, so she had it prepared to mark someone else than herself with it.
Gold mask embellished with delicate designs.
Amidst the Concealers who hole up in closed rooms and believe in the Soul Arts, the wearing of this is permitted only for those of especially high rank, it is said.
But, who are these women? Where does this sect originate? Mephistopheles wears the Foe's Ring, wields the Talisman of Beasts, and casts the Soulsucker spell. These are all items relating to the First Scourge, as they allude to the Old One and its power to continuously, maliciously seize souls into oneself. Thus, the founding members of the Secret Society did so during that catastrophe, probably being some of the witches of the Nexus even, and after the ban of Soul Arts, they went hiding.
Red-black mystical ring bestowed by Mephistopheles. Attack power when one has become a black phantom is increased.
This mystical ring supports the “bestial connection” which attacks warriors from other worlds and therefore makes them one’s own sustenance.
It seems like a Friend’s Ring that forms a pair with this one exists.
They desired to keep practicing magic, mirroring the now-sealed "God" of Mird, teaching the Soulsucker spell and inheriting those items through generations. It is no coincidence that the Gold Mask yields us a Colorless Demon's Soul by trading it with the Double-Winged Hawk Girl; and Mephistopheles does give us Colorless Demon's Souls as part of her rewards.
Yet to do what they wanted, they had to hide and do it in secret. And where to hide, if not on the once-magic-rival of Mird?
Because this sect is female exclusive, and these women really hold no value for fellow life, it wouldn't be out of the table that, through their connections, they get a nobleman to get them pregnant, and once a girl baby is born, they hire an assassin to dispose of the father. Mephistopheles does make use of Yurt and later ourselves to kill her targets, after all.
Not only are they traitorous among themselves, but they also target anyone who might have knowledge over Soul Arts or could even oppose them, as Mephistopheles proves with the list of target given to us: she doesn't just target miracle and spell trainers, but also Patches and Ariona, and Ariona's bodyguard, Biorr. The likely reason as to why Ariona is targeted is because his father brought the reinassance of Soul Arts in the first place. Indeed, the fire magic of the Epee Rapier is just one of the reasons of why they're terribly feared.
The purpose of these misdeeds is that they want to be the only ones with knowledge over Soul Arts. The first time that Mephistopheles gives us a mission, she posits it as a chance to "eliminate a competitor", and when she readies herself to kill us, she states that "the secret of souls won't leak".
"How about it? If you think about it as eliminating a competitor, you don’t really need more of a reason, do you?"
"After that if you just die, the secret of souls won’t leak…"
This does go in line with how they are called in the Japanese script, for 秘匿者 not only doubles to mean that they conceal themselves, but also that they conceal something. In other words, the Secret Society hide themselves and hide Soul Arts so that only they can use them.
We can loot a corpse with the Gold Mask and the Epee Rapier inside the Ivory Tower of Latria, which implies that, when the Queen reopened it during the reinassance of Soul Arts, they assisted to it, maybe even to Yormedar itself, to find more targets for their affairs.
Nonetheless, during the Second Scourge, it seems that the Old Monk (黄衣の翁, "Yellow-garbed Old Man") has wiped out most of them, using them for the demon in the shape of a colossal flesh ball he seeks to create; and only Mephistopheles survived. Not that she cares.
Special thanks to Jon_dArc for accurate JP text translations.