From Destinypedia, the Destiny wiki
Lost Sectors are a type of PvE activity that were introduced in Destiny 2. They consist of a hidden dungeon or side area that contains a large number of enemies, a boss enemy, and an unlockable treasure chest. Lost Sectors can be identified by a symbol that appears near its entrance, but the difficulty of actually locating the entrance will vary. Killing the boss inside the Lost Sector unlocks the chest, which will always contain some Glimmer, at least one piece of equipment and a Token for that planet. With update 2.0.0 and the release of Forsaken, Lost Sectors no longer grant Tokens upon looting. Lost Sectors can be completed an infinite number of times and automatically repopulate after being exited.
Legendary Lost Sectors
Legendary Lost Sectors were an activity available during Season of the Worthy. They were accessed through a console situated next to Rasputin's screens within each Seraph Bunker once that bunker was cleared for the day. Offering significantly higher difficulty than regular Lost Sectors, they featured modifiers and Champions, similar to Nightfall: The Ordeal.
Legendary Lost Sectors returned in the Season of the Hunt. They are accessed through a flag, similar to the ones used to enter adventures and campaign missions. Similarly to their previous iteration, they had higher difficulty and modifiers, as well as awarding Enhancment Cores (and an Exotic Armor of a specific slot, if solo) on completion. Master Lost Sectors were also added, being even more difficult than their Legendary variant. Completing them alone, which is quite a challenge, is the only way to get the new exotic armor pieces from the Beyond Light expansion.
List of Lost Sectors
European Dead Zone, Earth
The European Dead Zone holds 16 Lost Sectors.
New Pacific Arcology, Titan
The New Pacific Arcology holds 3 Lost Sectors.
Arcadian Valley, Nessus
The Arcadian Valley holds 5 Lost Sectors.
Echo Mesa, Io
Echo Mesa holds 3 Lost Sectors.
Fields of Glass, Mercury
Fields of Glass holds 1 Lost Sector.
Hellas Basin, Mars
Hellas Basin holds 2 Lost Sectors.
The Tangled Shore, The Reef
The Tangled Shore holds five Lost Sectors.
The Dreaming City, The Reef/Ascendant realm
The Dreaming City holds 3 Lost Sectors.
Ocean of Storms, Moon
The Moon holds 4 Lost Sectors.
The Cosmodrome holds 2 Lost Sectors.
Rathmore Chaos, Europa
Rathmore Chaos holds 3 Lost Sectors.
List of appearances