Who's This? An adult Legion villain.
How you could have heard of him: Alan Moore being who he is, you may have seen Cosmic King's adult LSV appear in "Whatever Happened to the Man of Tomorrow?". His most recent appearance was in the Retroboot era, a story we're about to get into.
Example story: Adventure Comics #529 (October 2011) "The End" by Paul Levitz, Geraldo Borges and Marlo Alquiza
Transmuters are often nerfed so that they can't affect organic matter (for example), but when villains AREN'T, it makes them way too powerful and dangerous, and therefore must be used sparingly. In this case, his lower-powered decisions at the start of the fight are also less than convincing. No matter, I guess, because (and I hadn't remembered this going in) he is killed in the issue, taking Variable Lad with him (or vice-versa). I guess this really is "The End". For Adventure Comics, for Variable Lad, for Cosmic King, but also for the whole Adult Legion timeline!
Who's Next? A shadowy organization.