Answer by taking a quote from the following article:

Sage, also known as Tessa, is a fictional character in the Marvel Comics universe. She has most often been associated with the X-Men and the Hellfire Club, whom she spied upon for Professor Charles Xavier. A mutant, Sage possesses a number of mental abilities and was originally presented as the personal assistant to the Hellfire Club's Sebastian Shaw, but an extended retcon revealed that she was one of the first mutants discovered by Professor Xavier. She has been a member of the original X-Men teams, Excalibur, the Exiles, and a cross-dimensional X-Men team similar to the Exiles known as the X-Treme X-Men.

For reasons yet unknown, Sage leaves Sunspot's side and travels to England to join forces with the newly reformed New Excalibur. She helps them on various missions such as battling the Shadow King, and returning to ancient Camelot to prevent it from being prematurely destroyed. In a more subtle use of her observation abilities, Sage is instrumental in her teammate Nocturne's recovery from a debilitating stroke.  In order to learn more about a mysterious new foe, Albion, and thwart his attempt to conquer Britain, Sage creates the new cover identity and persona of Diana Fox. This persona appears to be fully devoted to Albion and his goals. However, this bothers her teammate Wisdom as he fears she might fail the mission, similar to her time with the Hellfire Club. Her Diana Fox persona climbs up the rank of Albion's Shadow Captains as Britannia, and develops a rivalry with Lionheart. Like the rest of Albion's army, Sage is given a suit that emphasizes loyalty to him, as well as equipping her with flight and superhuman physical strength. Although she has the opportunity to kill Albion, one purpose of her infiltration, she is struck by an errant pulse from an ancient item used by Albion to extinguish power in England. Though Lionheart suspects her betrayal, Albion believes she was only trying to protect him.  Wisdom's fears are realized after this when the Diana Fox persona takes complete control and Sage becomes fully committed to Albion's side. During this time, she slays two of New Excalibur's former allies in Shadow-X, Dark Cyclops and Dark Beast. A direct confrontation with her teammates in New Excalibur helps reassert Sage's original personality. Together with New Excalibur, Shadow-X, and now Lionheart, Sage defeats Albion and his Shadow Captains.

What happens after this persona is released?
Her Diana Fox persona climbs up the rank of Albion's Shadow Captains as Britannia, and develops a rivalry with Lionheart. Like the rest of Albion's army,