I thought they were built by them to mark their lairs, but doesn't seem so. Their size and shape, spiked and strangely segmented, don't seem to fit with any TES creature that I know of. And when you battle Mannimarco, some of these come out of the floor, complete with animation and everything, forming a ring to hold you in there until you defeat him. So I was wondering that necromancers were granted some sort of "bone magic" powers, a la Diablo II, by the God of Worms.
Well, maybe comparisons to Diablo II are hideous, but its the only thing I could compare to. Is there any info about these bone structures?