Liesa

Liesa, the Shroud of Dusk, was an archangel from Innistrad. She and her entire flight were killed by Avacyn after they sided with a demon lord a millennium ago.

The Four Sisters
Liesa, Bruna, Gisela, and Sigarda were angelic sisters who pre-existed Avacyn. The four archangels sisters defended humanity: Sigarda, who protected the living, Bruna, who protected the dead, Gisela, who fought monsters, and Liesa, who consorted with them.

Liesa engaged in unconventional actions and made unwelcome allies. Sometimes she would consort with vampires and witches, even demons and devils. "We must know our enemies if we are to defeat our enemies", she would say. She was often distrusted and disliked by the other angels, even sometimes by her three sisters. But the four of them had a deep bond, and though Liesa charted a very different path, still she was their sister.

Until she formed a pact with a demon lord, an act condemned by them all. Avacyn had declared her a heretic, an accomplice to the very monsters Avacyn and all the angels were sworn to defeat. The three other sisters had agreed with Avacyn, but did not join Avacyn in her crusade against their dark sister. Avacyn had not needed their help. A thousand years ago, Avacyn single-handedly destroyed their sister and her entire small flight, and they had made the very mention of her name forbidden.

Trivia

 * Although not confirmed by official sources, by inference from the naming pattern of Innistrad's archangels (Blade of Goldnight, Light of Alabaster and Host of Herons) as well as Brisela, Voice of Nightmares and their flights of angels, Liesa's flight may have been the Flight of Dusk.
 * Sigarda described her as the oldest and the most powerful of the four archangel sisters.