The Legacy Chain are a hero group in the now-defunct City of Heroes MMO.
The Legacy Chain has its origins in druid mysticism. It is a secret sect of mystics that are dedicated to preserving the secrets of the arts, and are willing to sacrifice everything and everyone, even themselves, to protect those secrets.
The organization is broken into several disciplines that comprise the essence of positive Druid magic: Steel, Light, Flame, and Earth. Each pledged member must train and master that discipline before they can take an oath to serve Legacy Chain.
The group focuses their efforts on the Rogue Isles because that is where Mu Mysticism is practiced the strongest, and it is their belief that their purpose there is to maintain a balance of light magic to counter the dark essence of Mu.
Legacy Chain were also the ones that discovered the dark link between Ghost Widow and the Rogue Isles, and have been doing everything in their power to make sure the dire prophecy of the Widow's demise does not result in the complete destruction of the Isles themselves and every being there.
(See their Paragon Wiki entry for more information.)
Legacy Chain was featured in the "Limelight" storyline in "Guardians of the Dawn Spotlight" comic series. They were the ones that discovered the link to Ghost Widow and Midnight Arachnia, and, in fact, were the ones Midnight Arachnia was working for so she could spy on Arachnos.