Once you have all the ithildin fragments in an area, you can attempt to unlock the Ithildin Door. In Minas Ithil, this is in the same barrow where you learned Elven Rage from Cel's memories. Place the missing words like so- Shadows, Drums, Cadence, Land, Wrath, Doom.
This will open up a vault with a legendary item. Choosing whether to raid these vault now or later is something of a choice. The legendaries are always attuned to your current level, so they'll be more powerful later on. Conversely, while you'll have several ways to get legendaries later, this is one of the only ways to get legendaries in the early game.