Actually MXYZPTLK is quite prominent in the Superman myth when you look at the comics. He also appears in Smallville. So I wouldn't think of him as strictly a L&C villain.
Tempus, on the other hand, is the only villain that is specific to L&C that comes to mind immediately.
So I'm with Jensguy, I'm guessing you are referring to Tempus. Either way - I think your idea sounds great so far and I'm looking forward to hopefully reading it!