This here is the What Ancient Mystery Card. Its an octopus holding what appears to be radianite. We see some black substance curling up its tentacle into the shard.

So you must be wondering - what the almighty flying frik does this have to do with Omen.

Well, first of all, notice the black substance which is remarkably similar to that of when Omen uses his ability. We know that there is a possibility that Omen isn't human - he could be anything, and what better than an octopus! If humans could gain power from radianite, so could other creatures. And octopuses would make a great candidate, being in my opinion the most remarkable and intelligent creatures after humans and dolphins. They have learning capabilities of a human child. Why couldn't Radianite have granted it the power to take any shape, and live for longer, and do all the other cool stuff? Besides, if you look at other Radiants, you'll notice that while they do gain power from radianite, nobody has been changed nearly as much as Omen, so other creatures could possibly be affected more strongly by the substance. And then let's take a look at his face, or at least what we can see of it. Look closely. Thinl carefully. How could he have those 3 slips if they were not tentacles covering a source of energy within? What else could it possibly be?! Now you could tell me that I've just been rewatching Assasination Classroom a bit too quickly but I can say with confidence that while that may be true it is not impairing my judgement in the slightest.

This post was made with more comedic intent than anything, but i do still believe that this is very much a possibility. Yes, it seems farfetched, but after reading this, can you say with full, 100% certainty that this is impossible?

Edit: thanks for all the comments! Here are some additional things I saw in the comments that would be valid.

Omen has the ability to blind his enemies, be it with cloud or paranoia. And what else has the ability to blind its enemies with cloud and paranoia? Thats right, octopuses. Not to mention all his abilities are black; not just black, no - theyre an inky black BOOM OCTOPUS INK. And if you notice his abilties as well as his design incorporates a tinge of blue. What else is blue? Octopus blood.