The original art description for Cruel Ultimatum said this:
Location: Grixis, inside a towering necropolis, in a huge room made of blackened skulls and bones of all kinds
Action: Show something no one would expect to see: a great, winged demon lord . . . cowering in a corner. Maybe's he's even clutching a gut wound that oozes black blood. On the walls around the cowering demon we see the huge shadow of a figure whose shape is distorted by the skulls and bones of the room's walls. The shadow reaches an outstretched, clawlike hand toward the demon.
Focus: the scene
Mood: A creature of supreme evil dominance, brought low and terrified.
Notes: The shadow should suggest some kind of enormous humanoid dragon or dragon/demon hybrid. It's meant to be a very subtle clue/teaser for a character that players won't see until later.
Here's the sketch that Ralph Horsley turned in:
The final version had the shadow downplayed to make it more of an Easter egg. Mouse over the final art to see the shadow!
And here's the final card!