It’s in the Bonestrewn Crest point of interest area. Start at Wittestradr Wayshrine and follow the road going north. The road takes you straight to Bonestrewn Crest, but just before you reach it, you will see torches dug into the ground and a huge mammoth skull with tusks next to them. The skyshard is lying between tusks.
Central Eastmarch, northwest of Wittestadr Wayshrine, northeast of Bonestrewn Crest.