A Mantle, or Mantle of Power, is a magical phenomenon that acts as a moveable container for the power and persona of a supernatural being. Not every supernatural being has a mantle; mantles are typically associated with magical heavyweights like pagan gods or the Queens of Faerie. A being in possession of a mantle assumes the identity it contains, and can wield its power. Mantles are first mentioned in Summer Knight.
Nature of Mantles
A supernatural identity with a Mantle may be thought of as a kind of position to be filled. Only one person can hold the position at a time, but many people can hold it in succession and each occupant of the office is granted the same authority.
When a person assumes a mantle of power, the mantle begins to influence the personality of its host. Harry Dresden's experience with the mantle of the Winter Knight began as emotions and urges that continuously built in intensity. In the mantles of the Queens of Faerie, this effect is extreme: the personas in these mantles will completely overwrite the host's original identity. According to Bob the Skull, after wearing the Mantle of the Summer Lady for about a decade, Lily would be virtually indistinguishable from her predecessor, Aurora. This makes the owner of the mantle expendable, reduced to the status of an interchangeable part.
It is possible for a single individual to possess more than one mantle, although it appears that only one can be worn at a time. Donar Vaderrung usually wears the mantle of the Norse deity Odin, but also spends time in the identity of Kringle - a pseudonym for Santa Claus. The mantles of Sidhe Knights give their bearer powers in line with their corresponding Faerie Court. However, the bearer risks losing access to the power of the Mantle if he defies the Law of the Court.
A Mantle can be transferred from one being to another, usually through the death of the current bearer. Once released, the Mantle seeks out the nearest "reflection of itself" or the closest appropriate vessel. Should there be no viable vessel, at least in the case of a Sidhe Knight, the Mantle's power returns to its corresponding Queen to be bestowed upon one at a later time. The Mantle of the Wild Hunt is one case where the Mantle can be passed without requiring the death of the bearer. Harry Dresden took it from the Erlking after defeating him and Kringle in combat then later returned it.
In the series
In Summer Knight, Bob states that the Sidhe probably don't care about Ronald Reuel's murder, they only care about the Power. Harry Dresden informs Murphy about the case, that he must find Rueul's murderer and restore the stolen "magical power thingie", as Murphy phrased it.
Aurora's mantle passes to Lily when Dresden kills her on the Stone Table. As Lily says: the "power flowed onto the nearest Summer Vessel". Lily then passes on the then-vacant Summer Knight's Mantle to Fix.
In Cold Days, Bob explains to Dresden that it doesn't make sense that Mab would want Maeve killed since the Mantle would simply pass on to another who would become another "Maeve" as the Mantle changes that person who wears it. Thomas Raith advises Dresden not to ignore the predatory impulses the Winter Knight's mantle causes and to acknowledge them instead in order to be in control of them and not allowing them to sneak up on him.
The instant that Dresden said: "So, screw Winter Law—", the Mantle disappears. Dresden is filled with intense pain and fatigue, followed by his body going completely numb from the waist down and he could no longer hold himself up or control over the functions of his lower body, his spinal injury returning. So, not only is the Winter Knight's power taken away but his body is returned to the state it was in prior to taking on the Mantle. It comes back a moment later.
Kringle met with Harry Dresden and Mab at MacAnally's to discuss how to deal with Nicodemus Archleone' plan to break into Hades' vault and his eventual betrayal when the heist succeeds. Dresden asked why he came as Kringle and he explained that Donar Vaderrung and Kringle are legally separate identities residing in the same body. As Kringle, he would answer Mab's summons promptly, but would have told her to get in line if she instead asked for Donar Vadderung.
In "Cold Case", the Winter Lady's mantle is shown both to increase the wearer's sexual urges and violently prevent any action on them - when Molly Carpenter and Carlos Ramirez tried to share a night together, she badly beat him up under the Mantle's influence.