Well, my beliefs about Nimue are pretty much cemented now. 2nd Dark One probably. Only…how will they find her if Dark Ones are usually killed to pass on the mantle?
I’m going to venture a guess that individual DOs are not in fact dead as such, but rather their souls become part of the Darkness and are stored somewhere (in the dagger, the vault, or some other pocket universe) until the reforged Excalibur somehow releases them back to a physical plane. I think Rumple’s whole heart drama in the previous season hinted at a process of progressive corruption where the host eventually becomes ALL darkness – my guess is that when they reach that point they more or less lose their mind or personality altogether, and become sort of individual embodiments of the (general) Darkness. Something like that.