The Dark One is time itself.
This theory was proposed by Richard Fife during the discussions and he goes into detail about it on Theoryland. The gist is that the Dark One is actually the Great Serpent itself, the Great Serpent being a representation of time.
The symbol of the serpent and the…
A very interesting theory, and quite probable.
Though I do have to wonder if this theory means that the Dark One is Time in a metaphoric sense or a literal one.