If a RAID: Shadow Legends clan leader is inactive, they will eventually be automatically replaced, but the exact inactivity period is not specified publicly by the developers. Some sources suggest it could be around two weeks, while a more recent user report indicates a longer duration, and others note that in other similar games, inactivity can be longer (e.g., 90 days in Diablo). You will have to wait for the game's automatic process to trigger because there is no feature to manually remove or replace a leader in the game. 

What to do

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Wait for the automatic process: 

RAID: Shadow Legends will eventually promote another member to the leader position after a prolonged period of leader inactivity. 

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Encourage activity: 

Ask other clan members to log in regularly to keep the clan active and increase the chance of being selected as the new leader when the rotation occurs.