The Black Lantern Corps has made life very difficult for anything living. We got the first inkling of their arrival way back in Green Lantern #6, when Black Hand dropped mention of the Blackest Night. It sounded like he was spouting off. But it was promised on the final page of the Sinestro Corps War event that a Blackest Night was indeed coming. We saw the scene in which a mysterious voice ordered the dead Anti-Monitor to rise and become the battery for the Black Lantern Corps. But who is behind all of this? It's not the Anti-Monitor himself -- the voice ordering him to rise was speaking when he was dead. And the splash page in Blackest Night #0 says that the Black Lantern's "ultimate purpose and its creator are unknown."
I love a good mystery. In this case, there are perhaps no clues in the usual sense. We're not going to see a muddy footprint and match it with the villain. I think, however, that Geoff Johns has made some decisions in his narration that make more sense if a particular (and fairly obscure) character is filling that particular role. I think the leader of the Black Lanterns is the brilliant creation seen in one Alan Moore story which has undoubtedly inspired the story that Johns has woven. My guess is that the leader of the Black Lanterns is the demon known as Qull.
We know through Atrocitus that the Five Inversions were the ones to realize that Blackest Night was going to happen. Atrocitus found William Hand on the basis of that prophecy. And the Guardians know of it via Abin Sur, who was told of it by Qull. Qull was obviously the smart one of the bunch. Because of what Qull says, Atrocitus (who is obviously not the smart one of the bunch) kills him. Would Qull not have seen that coming? Qull had eerie prophetic accuracy in at least a couple of ways in dooming Abin Sur and foreseeing the Green Lantern careers of Hal Jordan and Sodam Yat. If there's any logic to it, Qull wouldn't have set that path in motion if he didn't stand something to gain.
Qull dies in a flashback that opens Green Lantern #28. This event is dated to the time that Hal Jordan was a rookie. A series of questions:
Why, if you're Johns, kill the one interesting character from the Five Inversions, and make up a new one to represent them?
Why put Qull's death on-panel? Why, when the plot coming up is about dead characters coming back to life... Why open #28 with a panel staring right into Qull's eyes?
Why include the detail that Qull's "recklessness" left the Five Inversions on Ysmault instead of being imprisoned in science cells on Oa? How can a prophet be so reckless?
Why include Qull telling Atrocitus that he (Atrocitus) is a greater fool than Abin Sur? The splash page defining the Red Lanterns says that Atrocitus's prophetic rituals were "primitive". Whose prophetic rituals were advanced?
It's not just that Qull's actions are illogical for such a sage, but that Johns seems to include details that would have no other role unless this was a crucial part of the story. More so than just giving us the leader of the Red Lanterns in Atrocitus. And notice that in Green Lantern #43, William Hand hears the black-ballooned voice giving him orders immediately after first coming upon the weapon that came from Ysmault.
I'll throw down my guess now that Qull, he of the 19-week laughs, set this up as his bid to get out of captivity. By killing him, Atrocitus made himself the leader of the Red Lanterns. If my guess is right, it also made Qull the leader of the Black. Knowing that the Black will "win" the color wars, he sees his eventual victory and domination of the universe, and now his bid for that role has started the Blackest Night underway.
Edit: The suspense is over, and I missed the obvious. Geoff Johns has given us the answer in an interview way before the story gets there: Nekron is the Big Bad. Did Qull have some ace up his sleeve when he died? It seems not.
By giving us the answer now, Johns tells us first and foremost that the story is not a whodunit. Nekron was more ugly than scary in his earlier appearances. Johns is going to have to ramp the character up to make Nekron deserve this kind of role. And that's exactly what we should expect.