I was thinking a "solid", but semi-transparent shadow horse, with blackish mist rising from it's body. Through the transparency you can see inside the horse, the tormented faces of all the people killed and sent to Sithis. While the horse is out, the cries and pleading can be faintly heard. It's hoofs would be shrouded in dark fog/mist. When the horse is summoned, the player hears a faint whisper that says, "Hail, Sithis". And the horse slides in from a shadow on the ground. The neighing would sound like a slight roaring echo - quite scary.
Motto: Make deceivers believers.
Strength of character is not a physical thing. -EWalking a mile in someone else's shoes, has nothing to do with the path taken. -E
An accusation of elitism, is an indirect recognition of one's own inferiority. -E
The best way to prove someone wrong, is to do better yourself. -E
I keep forgetting to remember to get a photographic memory.