I found one when a stone started shooting arrows at me.
I could have just left the area but was bugged enough to dig it out with a pickaxe. (While timing shield blocks)
There was a skeleton archer embedded in/below the stone and beside the tomb opening. About an hour after clearing the tomb, while randomly trotting through the forest, I had the thought: "I bet I could have just slammed the rock with Stagbreaker a few times."