Shadowmere is Astrid’s personal horse and is received during The Cure for Madness, a Dark Brotherhood quest. Shadowmere has more health than other horses in Skyrim and the ability to regenerate health extremely quickly.
Shadowmere’s introduction is rife with macabre undertones: rising up from the bubbling dark waters, with black smoke, a dark aura, and of course glowing red eyes. Astrid claims that the horse is “One of us”.
Shadowmere is capable of being killed. However, 15-20 days later, Shadowmere will respawn in the location of death.