The Seven Lights, or "Seven Guardians of Light" is small group of Keyblade wielders who are destined to oppose the 13 darkness and create the X-blade in the process. Currently, the only information known is that Sora, Riku, and Mickey are going to be members, however the other four are as of yet unknown.
During Dream Drop Distance, Master Xehanort revealed that he knew his attempts to create X-Blade through the reunion of Ventus and Vanitas was a hasty attempt and that he had always been searching for a true way to forge it. For this reason he enticed Maleficent into capturing the Princesses of Heart, with his Heartless Ansem overseeing her progress, while his Nobody Xemnas created Organization XIII so his splintered selves can obtain the Lights and Darknesses, respectively. But, as Ansem failed to possess Riku and Roxas returned to Sora, Xehanort's new organization would be one member short so he decided to use Sora. This attempt, too, failed thanks to Lea, Donald and Goofy.
As the 13 Darknesses return back to their timelines while short one member, Xehanort tells his opponents that the battle between his Darknesses against their Lights will happen and they they will meet again at the fated place. Because of this threat, fearing Xehanort would go after the Princesses of Heart if they did not go through with it, Yen Sid and Mickey resolve to gather the Seven Lights that would be composed of Sora, Riku, Mickey, and Lea. Though Mickey thought that Aqua, Terra and Ventus could fill the membership once found, Xehanort revealed Terra to have become his vessel with the implication that he is among the Darknesses. However, Yen Sid had Kairi brought before him to teach her to wield her Keyblade.