Tʜᴇ Eʟᴅᴇʀ Sᴄʀᴏʟʟs V﹕Sᴋʏʀɪᴍ | ᵃᶰ ᵃᵈᵛᵃᶰᶜᵉᵈ ʳᵒˡᵉᵖˡᵃʸ discussion

Darkwater Pass is a cave.

It can be found by traveling from Windhelm to Fort Amol, and from there following the path southward to Darkwater Crossing. The entrance can be found next to a waterfall roughly southwest of the Crossing.