This also includes the vault located inside the Ragged Flagon - Cistern, home of the Thieves Guild. This artifact is how Mercer Frey is able to break into many places with ease, such as Snow Veil Sanctum or Irkngthand.If the Dragonborn serves time in jail, the single lockpick taken with them to jail will be the Skeleton Key.Keeping the key effectively renders the Lockpicking perk Unbreakable entirely obsolete, as well as most of that skill tree.
The Skeleton Key can be used like a regular lockpick and can be kept indefinitely, provided it's never returned to the Twilight Sepulcher, during the mission ' Darkness Returns.' In addition, the Lockpicking skill will still increase when a lock is opened (removed in patch 1.4, back by 1.9). In order to complete the remaining quests for the Guild, the Skeleton Key must be returned in the Twilight Sepulcher.
It can be obtained from Mercer Frey at the end of the Thieves Guild quest Blindsighted.