I'm beginning to think that Tu must have evolved, along with Tevildo, into Sauron eventually.
I found the passages relating to Tu and to Nuin the Dark Elf (which aside from anything else, proves how your eye can always be caught by something anew in Tolkien's work - it's a quite beautful piece of writing and a crying shame he didn't use it
), and what strikes me most strongly is that Tu reminds me of Merlin, what with the dwelling under the lake, and the passageway leading to the sleeping Men before their awakening.
Tu doesn't really strike me as having been evil though, and as Groin says, Tolkien struck out the possibility of this in one of his drafts - so could he
really have been the genesis for Sauron???
He certainly seems like a prototype Maia though, with his knowledge of who/what the sleepers were, and his warning to Nuin not to tell any other Elves what he saw.
The sleepers themselves struck me as being very Arthurian...