succession to the throne
嬗 centers on the formal transfer of royal authority: succession to the throne. It is a specialized term for the passing of sovereign power.
嬗 combines 女 (woman) with 亶, which likely contributes the sound and may relate to the idea of transmission or change. The exact historical development is uncertain.
A woman (女) beside the elaborate 亶, as if she is the one passing the crown in a royal ceremony. The scene helps recall the idea of succession to the throne.
For セン, imagine a thousand (千, せん) courtiers bowing as the succession is announced: 千 -> セン, and the throne passes smoothly.