Mantle

A Mantle of Power, or simply Mantle, 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. A mantle can be transferred from one host (possessor) to another; this typically occurs upon the death of the current host. Mantles are first mentioned in Summer Knight, and become relevant again in Cold Days.

Nature of Mantles
A supernatural identity with a mantle may be thought of as like a governmental office. 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 delegated the same authority. When a person assumes a mantle of power, the mantle begins to alter its personality of its host. Harry Dresden's experience with the mantle of the Winter Knight began as emotions and urges that continously 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 Aroura. The owner of the mantle is expendable, reduced to the status of an interchangable 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. Donnar Vaderrung usually wears the mantle of the Norse deity Odin, but also spends some time in the identity of Kringle - a pseudonym for Santa Claus. The mantles of Sidhe Knights mantle give the bearer powers in line with their 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. When the bearer of a mantle dies, the mantle automatically travels to a new one. When Aurora was killed, the mantle of the Summer Lady moved to a nearby "reflection of itself" - the mantle of the Summer Knight, hidden inside Lily. It appears that if there is no nearby "reflection," the mantle looks for the nearest appropriate vessel. When Maeve killed Lily, the Summer Lady's mantle  was transferred to Sarrissa, whom Mab had been grooming to be Maeve's replacement.

The mantle of the Wild Hunt can be passed without the death of the bearer. Harry Dresden took it from the Erlking after defeating him in combat, and later returned it.

Summer Knight
In Summer Knight, Bob said that the Sidhe probably don't care about Ronald Reuel's murder, they only care about the Power. Harry told 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 passed to Lily after Harry Dresden killed her on the Stone Table. As Lily said: the "power flowed onto the nearest Summer Vessel".

Changes
In Changes, to become the Winter Knight Harry had to kill Lloyd Slate in the Stone Table in order to release the power, the Mantle, and return it back to Mab so that she could then pass it on to Harry.

Cold Days
In Cold Days, Bob explains to Harry that it doesn't makes 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 advised Harry not to ignore the predatory impulses they originate, to acknowledge them instead to as to be in control of them and not allowing them to sneak up on him.

The instant that Harry said: "So, screw Winter Law—", the Mantle disappeared. Harry was 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. He had broken his back when he made his agreement with Mab. So, not only was the Winter Knight's power taken away but his body was returned to the state it was in prior to taking on the Mantle. It came back a moment later.

At the end, on Demonreach, the Mantles for the Summer Lady and the Winter Lady changed hands—Lily's Mantle entered Sarissa and Maeve's entered Molly Carpenter.