Who's Krona?

Who's This? The guy who unleashed evil on the universe.

The facts: The pre-Guardians Oan (or is it Maltusian?) whose hubris led him to peer too far into the past to see the origin of the universe. Seeing the Giant Hand of God(TM), it caused anti-matter to spill out, splitting the universe into a multiverse (or at least, a positive and a negative universe, if we have fewer Earths to work with), and spilling evil into the young uni/multi-verse. It would force the Oans to feel obligated to police it, first with Manhunter robots, then with the Green Lantern Corps. And force DC to spend an inordinate amount of page space recapping those events - Krona appears in a LOT of comics, it's just that it's always the same scene. After all, they reduced him to disembodied energy as punishment, so he wasn't going to be around much. He used Alan Scott's ring to bring himself back in the mid-60s, but this resurrection was brief. He returned for a longer engagement in the Tales of the Green Lantern Corps mini-series in 1981, then from time to time as time went on - as Entropy in the third volume of Green Lantern and as the big bad of JLA/Avengers and Trinity. Krona returned in Brightest Day and was destroyed by Hal Jordan again in the War of the Green Lanterns event just before we were handed the New52. The Krona of THAT continuity started out as a much nicer guy, but made the same mistake and went bad. The Green Lantern stuff was too popular at that point for it to be rebooted very much, so all those later stories had to happen. His essence survived War of the GLs and after a story arc involving Atrocitus, he was finally destroyed by Volthoom the First Lantern.
How you could have heard of him: That dang flashback. You'll even see a version of him in the GL movie, GL: Emerald Knights, and GL the Animated Series. You just can't avoid Krona when discussing the DC Multiverse.
Example story: Tales of the Green Lantern Corps #3 (July 1981) "Triumph" by Len Wein (from a plot by Mike W. Barr), Joe Staton and Frank McLaughlin

Recap: The universe is caught in some kind of collapsing vortex that will grind it back to the primal atom. Nekron, Lord of the Unliving has sent Krona back to our reality where he has murdered a bunch of Guardians and Lanterns AND destroyed the Power Battery, somehow opening a portal between Nekron's dimension and our own. Now he commands an army of dead Lanterns. So yeah, it's Blackest Night v1.0, or at least, Geoff Johns's ur-text for his run on Green Lantern. As Hal Jordan rallies the troops, the Guardians try to keep the universe from shrinking. But Krona makes a b-line for them:

Nekron seems to have gifted him with his death energy (Black Hand's in more recent events), and he throttles Guardians to suck their vitality. And with each Guardian's death, the portal between Life and Death grows wider. And the wider it gets, the more undead armies can step through. But to what purpose?

Krona is obsessed with one thing: Seeing the birth of the universe, and it seems, ANY universe. So he's heading for a reboot of ours (is this where Parallax got the idea for Zero Hour?), one where HE can be the Big Hand(TM). But in this iteration, Krona's power is tied to Nekron's, and that's ultimately his undoing. When Hal starts to turn the souls of the dead into Lanterns, Nekron becomes distracted to the point of defeat, cutting Krona off from his deathly teat!

He's quickly sent back to the afterlife before the Guardians undo all the damage he and Nekron caused.

Krona as a piece of DCU history is his rightful place, and I'm not sure bringing him back as a villain is a good idea. But waste not, want not. And it's not uninteresting. He's a bit "one-note", but his one note is an epic one that can cause great damage to the timeline. A crisis, if you will. His part in history can always be invoked to make Big Changes(also TM) to the DCU, and I don't know if you've noticed, but DC loves to do that.

Who's Next? A super dog.