Mantling

Mantling is a process in the Elder Scrolls universe by which one entity becomes another entity by impersonating it. In most known or speculated instances of this process, the entity being mantled is a deity, while the person mantling is a mortal.

Merely impersonating someone publicly is not enough to mantle them. To mantle someone, you must become so like them that there ceases to be a functional difference between the two entities; it seems that at this point the universe itself ceases to distinguish between the two, and they become one entity.

It seems that while mantling does involve, to some extent, assuming some other entity's identity, this does not appear to neccessarily involve giving up the mantler's own identity, at least in full.

Origins
The concept of mantling is unique in that it has few, if any references to it in any major Elder Scrolls game, despite having been confirmed to exist in the universe by several developers, particularly on The Imperial Library. There are some books which reference events that could be mantling, however it has never been referred to specifically by its name.

Tiber Septim/Talos
Mantling may have been what caused Tiber Septim to become the deity Talos upon his death; he spent much of his life campaigning around Tamriel with the Numidium, which was originally constructed to draw power from the Heart of Lorkhan. While the Numidium in Tiber Septim's possession was powered by the Mantella, not the Heart, it is possible that so much time spent around and controlling the Numidium's power was enough for him to effectively mantle Lorkhan, and take his place among the Nine.

This example also supports the idea that mantling may be influenced by voids of power or identity in the universe; Lorkhan, while known, was not an active diety, and this absence of direction in his power might have made him more prone to being mantled. Understood another way, it may have been easier for Tiber Septim to mantle Lorkhan (Either intentionally or not) because the universe itself wanted a ninth diety to direct Lorkhan's power. (Though this would raise many questions about the nature of the power from the Heart of Lorkhan, and whether it affected or was affected by Tiber Septim becoming a diety.)