Death From Above by @drakhenliche (Isabelle Davis)

blurb (this probably won't make much sense because I'm very, very sleepy but I'll try and explain anyway): Those who hold power in Spirit World (or who have been deemed suitable to be gifted with one) ride Steeds known as slavekkes. These are not natural creatures and were created through magic. They are shapeshifters and transform into the most suitable form for the environment they are in, but all have a chosen form (which differs from slavekke to slavekke) which they remain in unless they have to change.

Though they are sentient and possess intelligence they are little more than automatons, They are always at least partly or fully armoured (this cannot be removed) and some appear as undead with rotting flesh or are completely skeletal whilst others look like fairly normal mounts save for the non-removable armour.

Ymmu, as right-hand to the Lord of the Undead, has a special slavekke called Ragnok. Ragnok is the most powerful of slavekkes and this is reflected by its chosen form being a dragon. It has also been imbued with an intellect above any of its kind and actually shows signs of personality.