Got mine real quick off Horror Guard on other side of bridge. Cleared every thought horror on east side with no luck.
Crossed invisible bridge, turned south. Before you get to the spot marked VT SHARD CAMP on the Brewall map, there is a spot with two HORROR GUARDS. One of those dropped it. Wasted time clearing east side horrors just to see if they do drop the shard. Not for me. Otherwise this would have required 15 minutes. Luck of the draw, though, eh? YMMV