A Key of Guidance is an item that can be used to reset a character's level to 1 when they reach level 99, in a process called soul sublimation. They will gain a star by their level and any limit break books must be used again, but this raises their stat caps and allows them to gain more stats as they level again.
Keys of Guidance can be obtained through most event exchange booths, and occasionally via other means such as login campaigns, quest rewards, and Valhalla Assault point rewards.