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Order of the Blackened Denarius
The Denarians are unions of a human host and one of the Fallen. There are thirty such Fallen, each bound to a tarnished silver Roman denarius which bears its respective Fallen’s sigil; the thirty denarii represent the thirty pieces of silver paid to Judas Iscariot, and may possibly be those very coins. Once a human has physically touched one of the Blackened Denarii, the Fallen bound to it is free to communicate with its new host, able to bestow knowledge, physical prowess, and magical ability, for as long as the host maintains possession of the coin.
If the coin is merely touched briefly and not actually picked up, a shadow copy of the Fallen is put into the hosts mind to tempt it to pick up the coin completely and finish the bonding. It is not nearly as powerful as the original, but even this mental copy is capable of extreme power, such as casting life-like illusions (for the possessed only). It also possesses all the knowledge of the original – a heavy temptation in itself. Once the coin is taken up, though, the shadow is no longer needed and vanishes.
The amount of influence a Fallen chooses to exert over its host appears to vary—in the case of Ursiel, the Fallen had completely taken over the host body, and kept the host’s soul tortured and trapped. In the case of Nicodemus and his Fallen, Anduriel, the two work in what appears to be a mutually beneficial partnership. It appears that the Fallen who dominate their host’s soul are less powerful than those that work with their hosts. This is most likely due to the issue of free will, since the ones that deprive their hosts of it are unable to draw on as much power.
In all cases, however, the free consent of the bearer is necessary to permit Fallen influence. Harry himself resisted the shadow of a fallen angel (Lasciel) for several years, drawing sparingly on its power only to ensure their mutual survival or when no other choice presented itself. This extended stay led the shadow to gain a measure of free will of its own, eventually sacrificing itself so Harry could survive a debilitating psychic attack.
If the Fallen has not completely overtaken its host, it appears that the host is able to consciously free himself or herself from the influence of the Fallen, simply by abdicating the power it provides and surrendering the Denarius. In the case of Quintus Cassius, a Denarian bonded with Saluriel, Cassius surrendered the coin to Knights of the Cross to avoid execution. Sanya admits to having once been a member of the Order, but he surrendered his Denarius freely prior to becoming a Knight of the Cross.
The Knights of the Cross were founded specifically to save people from the Fallen, and will almost always become involved (often by seeming random chance) in any situation that requires confronting one or more of the Denarians.
If the host of the Fallen is a wizard or magic-user of some sort, as most of those shown have been (although it is not clear if it happened before or after they became hosts), one way to be free of the Fallen is for the host to put aside magic forever and give up the coin.
Several of the coins are currently in the possession of the Knights of the Cross, the Church, and/or the Venatori Umbrorum. These coins are kept in heavily warded boxes in secret locations. However, Nicodemus and others have proven themselves adept at retrieving the coins, whether by assault or treachery, making the Knights’ task a never ending one.
Beginning in Proven Guilty, when evidence suggested that Hellfire had been used to assault the Winter Fay’s fortress of Arctis Tor, suspicion began to rise that the Denarians were in league with the “Black Council”. In Small Favor, it is revealed that Nicodemus, the Denarian’s de facto leader, was unaware of this, and that the Knight or Knights who have been aiding the Black Council are doing so without his approval (he personally considers the Council a threat to his own plans and in fact may be fighting against them).
Currently, the most likely suspect for the Black Council’s mole within the Knights of the Blackened Denarius is Nicodemus’s wife, Tessa, who may have stolen eleven of the thirty coins for herself when and fled, abandoning Nicodemus and his followers. Nicodemus himself is aware of this treachery, but was attacked and possibly killed by Harry before he could act on the knowledge. Jim Butcher has said that Nicodemus is alive, and he is now afraid of Harry.