Shadowmere is a black horse with red eyes that belonged to Lucien Lachance until she's given as a reward for completing the Purification. She is immortal and strong enough to assist in fighting multiple opponents. She can be used like a chest for storage if knocked unconsious but will use any items such as potions that have been stored on her. After recovering consiousness she will always return to Fort Farragut, where Lucien is/use to be.
Shadowmere returned in Skyrim under the ownership of Astrid until given to the player. If summoned near Shadowmere Lucien Lachance's ghost will greet her.