Blood Raven, first known as Moreina, is the corrupted form of the Rogue character from Diablo. She later appears as a boss and one of Andariel's and Diablo's henchmen in Diablo 2.
In her mortal life, Moreina was a captain of the Rogues in the battle against Diablo at Tristram; she, along with the Sorcerer Jazreth, joined forces with the Warrior Prince Aidan to penetrate the depths of the labyrinth and slay the Lord of Terror himself. Akara explains that she felt Moreina "brought an evil influence back with her", and that influence eventually led to Moreina's corruption into the evil Blood Raven by Andariel. The said influence was likely the corruption from Diablo's powers whom she and her friends not ready to deal with, that not only led to her own fall from grace at hands of Andariel, but also led Jazreth's fall to the dark side that turned him paranoid and became The Summoner whilst Aidan ended up embedding Diablo's soulstone on his head that resulting him became Diablo's vessel.
Kashya, leader of the Rogue Sisterhood Army and heroine, gives the heroes the Sister's Burial Grounds quest. Before Blood Raven turned evil, she was the captain of the Rogues in the fight against Diablo at Tristram. Herself and the sorcerer, Jazreth, joined with Prince Aiden and battled their way through Diablo's labyrinth and defeated Diablo himself. Akara says that she felt Moreina brought an evil influence back with her and the influence led her to being corrupted by Andariel.
Both good characters, Chasi and Kashya, mention that even though they were friends with Blood Raven, she must be killed to prevent any further corruption. This undead Demon moves at a quick pace while using attacks of fire arrows and summons undead creatures to aid her in fighting the heroes. During Blood Raven's death, her soul releases waves of lightning, which kill any nearby undead creatures when she is destroyed by the heroes.