Darien Gautier was a hero of the Daggerfall Covenant who discovered his true nature as Meridia's Golden Knight during the Planemeld. Brought into the world as a Light of Meridia through uncertain circumstances, Darien followed in his mortal father's footsteps as an officer of the Lion Guard. He would later join up with the Fighters Guild and helped lead their forces during the coalition invasion of Coldharbour, disappearing in the Planar Vortex in a confrontation with Molag Bal. Darien soon returned to Tamriel during the Daedric War as Meridia's vessel, learning that he was no Breton and possessed many otherworldly powers. He helped defend Nirn from Nocturnal's plot, sacrificing himself to restore Dawnbreaker
Darien remembers very little of his childhood, considering it all a big blur. He has a father, General Gautier, who was a high-ranking member of the Lion Guard, and does not remember having a mother. Darien would later discover that his mother was effectually Meridia, being brought into Tamriel as her vessel and last light. Before the Planemeld begun, he began having strange premonitions of Dark Anchors and Daedra invading Tamriel, each one ending with a strange light guiding him to safety. In 2E 582, his nightmares became reality when the Dark Anchors started to fall and the Planemeld proper had started. Opting to keep his prophetic dreams secret, he helped keep the peace in the Daggerfall Covenant the best he could. He defended Camlorn from Faolchu and helped thwart the plans of Angof the Gravesinger's Bloodthorn Cult at Cath Bedraud.
Darien wound up being one of the key players in the invasion of Coldharbour. During the final assault, he was shown holding back the Daedric forces alongside the Last Ayleid King while the Vestige and Gabrielle]] triggered the complete destruction of the Planar Vortex with the help of Meridia. Unfortunately, Darien was an apparent casualty of this battle, having disappeared when the Lights of Meridia were detonated.
Darien's apparent death was short lived however. In 2E 583, the Vestige found a strange note seemingly written specifically for them, claiming to be written by Darien himself, somehow alive and trapped within The Colored Rooms. When this note is shown to Gabrielle, she speculates that Meridia somehow saved him from the blast and that he is still alive, as his body didn't return to Coldharbour with them like Dynar's did.
In the events of Summerset, after a few failed attempts at communication with the Vestige, it is revealed that Darien is now working as a Knight of Meridia and has been gifted Dawnbreaker. He promised to serve Meridia and stop the plot of the Triad in exchange for his freedom and the ability to return to Nirn. Unfortunately, Dawnbreaker is corrupted by Nocturnal and Meridia's energies were stolen by the Triad soon after Darien's arrival, leaving him without orders and without a weapon. In the end, it's revealed what Darien truly is- the Last Light of Meridia and a container of all of Meridia's remaining energy. Knowing this, he sacrifices himself to release the energy and cleanse Dawnbreaker so that the Vestige can use it to put a stop to Nocturnal's plan once and for all. 
- For game-specific information, see the ESO article.