The Brothers like to play games, and their pieces are other sentient creatures. Once you have gotten their attention, you will be subjected to an endless stream of humiliation and public scorn. You will suffer embarrassment, deliberate degradation of everything that you value, loss of property, friends, social status, dignity. And it will not stop until you are dead or fled the corporeal plane; the Brothers in fact pride themselves on the number of individuals that they have driven to suicide, and treasure those incidents where they can get to the corpse first for one final jape. They are unencumbered by mercy, and not particularly restricted by the bounds of good taste. In short, they are vicious, sadistic bastards.
The Seraphim Council sometimes wishes that they had never approved this group's charter.
Purpose and Goals
Find demons and/or Hellsworn. Humiliate them to death. Repeat.
The Brothers (which have included Sisters since about 1895 AD) are mostly blessed souls; the group oscillates over the years between being a loose collection of groups semi-independently pursuing amusing projects and being a combination theoretical discussion round-table / coffee club. Currently, they are moving towards the former; unkind observers suggest that the group is primarily driven by the mutually-exclusive goads of laziness and boredom, and the Brothers haven't really been active on the corporeal plane since the 1970s. This was not entirely by their choice, but forty years is a reasonable amount of time for memories to fade, statutory limitations to run out, and countries to change leaders - and, in some cases, systems of government.
The few celestials that belong to the group are all Malakim. The Brothers used to accept other Choirs, until it became clear that the Council was becoming quietly frantic over the possibility that the Brothers might become a training ground for future Servitors of Dark Humor. Malakite Brothers tend to be either Judgment, the Wind, or (oddly) the Sword; their presence in Brotherhood operations tend to keep them from dragging on for too long.
Abilities and Resources
The Brothers look for a certain type of recruit: someone with a low sense of humor, a reasonably active imagination, and an uncritical willingness to do nasty things to genuinely nasty people. These traits (particularly the last) tend to make up for the otherwise relative lack of supernatural abilities. There is a surprising amount of trouble that you can cause when you can use your corpse for a prop... and still cause trouble afterwards; the Brotherhood makes it a point to explore the possibilities as fully as they can. Aside from that, the Brotherhood is a legitimate Heavenly group, which means that it has the usual Library lending privileges, access to the Groves and the Eternal City, is no less likely to get the brush-off from the Halls of Progress. Its Chapter House - located within sight of the Council Spires, at the direct order of the Seraphim Council - is in itself a fairly useful resource; over the last three centuries the Brothers have accumulated useful items for its activities, and they never throw out anything. What's there is not all that well categorized, but a walk through the Chapter House attic is rarely fruitless for the Brother looking for either equipment or inspiration.
Lastly: while no Archangel formally supports the Brothers - and not a few openly disapprove of them - their subordinates' subordinates are often a little more flexible. Or at least willing to try out one of the group's scenarios.
While no Archangel has actively lobbied to have the group abolished, roughly half of them forbid their Servitors to become Brothers. Oddly enough, both Dominic and Laurence permit it; the former apparently thinks that his Word's dissonance conditions can act as an effective check on the group (to be fair, it actually seems to do so), and the latter does not discuss the matter. The Archangel of the Sword does have a soft spot for humans, and the Malakim associated with the group do find many, many opportunities to creatively Smite evil, so he may have decided that on balance the group is worthwhile. Alternatively, the Angel of the Sword had a certain name for behaviors noticeably absent in the Archangel, so there may be a more earthy reason for his indulgence.
The Horde already has to deal with one set of malignant practical jokers with no respect for a proper demon's dignity, thank you very much: they have no need to deal with a second. That it might be sometimes useful to, ah, mistake the Brotherhood for Servitors of Dark Humor - particularly when they're unloading on a rival - is of course a scurrilous thought that should not be entertained by any self-respecting demon, and particularly should not be entertained by Servitors of the Game who might be tempted at the thought of setting up Kobal's demons to take the blame for a Brotherhood operation. Likewise, the obvious and natural antipathy that Fate has with, well, pretty much everyone else should not be seen as suggestive that there are many demons that might have an uncritical willingness to react slowly to Brotherhood operations that happen to target Kronos' organization.
The Brotherhood takes a detached view of the entire ethereal situation: they can't do much about them anyway (Blandine is one of the Superiors who bans her Servitors from joining this group). They will happily call in - or lead - a strike team against ethereals in violation of the Ban, if the ethereals are being nasty. If they're not, well, resources are finite, right?
It was by all accounts a shock for Jonathan Swift to discover that the achievement of his Destiny was due to the Bickerstaff Incident. Dean Swift was fond of practical jokes, and at the time the humiliation and embarrassment of the notorious astrologer John Partridge seemed merely enjoyable for its own sake. Apparently it was more than that: a John Partridge that had to spend the rest of his life explaining that no, really, he was actually alive was a John Partridge who was not called in to give some exceptionally bad advice at the coronation of George I of England. In other words, Swift's actions could be proven to have been of net benefit to the cause of Heaven.
The satirist naturally took this as a sign.
The Brotherhood's methods were relatively benign until about the middle of the 18th Century; it was about then that its members really started thinking of the implications of a post-mortem existence, and how it could be used for truly impressive displays of bloodily comic justice. It took about another sixty years for the death of the target to become the expected result of an operation, rather than a mild risk that one took. And now most of Heaven that cares is waiting to see what happens next. The situation is not being helped by the fact that the Brotherhood acts with the carefree assumption of ultimate righteousness that one gets when one has access to Absolute Truth and Universal Morality. If a mission is not in the best interests of Heaven - well, they can check that simply by saying that it is and looking to see if the Seraphim wince. If a mission is evil, the Malakim will stop it at the first reek of wrongness. So as long as those two criteria are met, what exactly is the problem?
Many angels find this argument logical enough in the abstract, but curiously less so in the concrete...