Seizing Power is an action that enables members of an Alliance to take control of the Alliance if the leader has gone inactive. The option first becomes available to officers after the leader has been inactive for 14 days, and then to any member of the Alliance after 30 days.
Once the leader becomes inactive, their name on the Message of the Day tab will appear as Leader name (missing X days). After the required number of days has passed, a 'Seize Power' button will be displayed. Whoever clicks and confirms will assume leadership of the Alliance.