Destiny 2 has several hidden areas within it known as Lost Sectors, and contain several enemies and decent loot to obtain. What can be more confusing, though, is the Legendary Lost Sector. While named similarly, it's a totally different thing, acting more like a mini mission that you can undertake.
How to access the Legendary Lost Sector
Accessing these legendary lost sectors require you to complete a few pre-requisites first. You'll need to complete the Raising Our Defenses quest, until the Seraph Bunker has been activated.
Once that's done, you'll find a console on the right side of the room that can be used to launch the Legendary Lost Sector. This is a tricky mission that isn't for the faint of heart, so you'll need to be somewhat experienced before continuing.
You'll be rewarded with some Warmind Bits to upgrade the Seraph Bunker once the Legendary Lost Sector is completed, too.