Kíli the Resourceful

If your third legendary, Saga, or artifact permanent enters the battlefield and then one of those permanents leaves the battlefield immediately afterwards (most likely due to the "legend rule" or due to being a creature with 0 toughness), you get an enduring story before it leaves the battlefield.

If you control three legendary, Saga, and/or artifact permanents but don't control a permanent with storied, you don't get an enduring story. For example, if you controlled three artifacts, lost control of two, then cast Thorin Oakenshield, you won't have an enduring story.

Once you have an enduring story, you have it for the rest of the game, even if you lose control of some or all of your storied permanents. The enduring story designation is on the player and can't be removed by any effect.