So what is this grave? According to the gravestone, Prince Arthas Menethil's steed was buried here. The horse, named Invincible (which was apparently a lie), seems to have been dug up recently. By the Scourge, or by someone else? Probably the Scourge, but anything can happen. It's fantasy after all. Drunken Dwarves might have dug it up on accident thinking it was the skeletal remains of a Titan or some nonsense.
I look forward to seeing what comes out of this, because the words on the gravestone don't give much away. It's very short and sweet, telling you what's there and little more. I guess that makes sense, It's a gravestone/memorial and I doubt the Scourge would spraypaint "ARTHUS WUZ HERR" across the stone. If you want to see what's engraved upon it, just take a look behind the link below.
[ Thanks for the tip, Joseph! ]