Recover or Use Duplicate Key(s)
This is a Daily Quest in Lord British's Castle (Invasion). To start the quest, talk to Captain Marisha, who is usually standing behind a barricade just inside the castle walls. She will tell you to collect the Brittle Copies of the Vault Key.
The keys can be found in sacks, on dead bodies, or on killed creatures within the Lord British's Castle zone. There are 3 different variations of this key numbered from 1 to 3. In order to obtain keys 2 and 3, you must to progress further through the progressively harder wave battles.
These keys can either be returned (as part of a Daily Quest) to Captain Marisha for a reward or they can be used to enter Lord British's vault.
If you return Key #1, the reward is a Ring of Jones, Common, gold, and 25,600 experience points (or 51,200 during double XP). If you return Key #3, the reward is a Belt of the Brutal Butcher, Common, gold, and 40960 experience points (or 81,920 during double XP). If you return Key #3, the reward is a Belt of the Patient Harvester, Common, gold, and 53,248 experience points (or 106,496 during double XP)
Journal Entry
Outside Lord British's Castle, Captain Marisha told me a watch commander was killed and his vault keys were duplicated by the invaders. If I find any duplicates of these three keys, she wants me to bring them to her. The captain specifically warned me against using them to enter the vault.