∞ BCE - Krona views the hand of the creator first forging the universe. His presence corrupts reality with entropy, which corrupts Oan energy with Anti-Oan energy. He dies in the big bang.
50,000 BCE - Krona is instrumental in the colonization of Oa & the discovery of Oan energy, a manifestation of life energy that permeates the fabric of reality, granting the Maltan settlers of Oa immortality.
24,000 BCE - Krona violates Oan law, using Oan super-science to travel to the beginning of time where he inadvertently corrupts all of creation with entropy.
Krona is SUCH a bizarre character. It's more or less impossible to have a story featuring him that doesn't upend the entire history of the universe. There might be mediums that could support this level of navel-gazing world building lunacy, but I really don't think anything but comics could do it this effortlessly.
Krona's Comic History
Krona appeared for the very first time in 1965, in Green Lantern #40. He's a product of a larger focus on world-building the Green Lantern mythology and the backstory of the Guardians. Krona was a Guardian that went back in time to the beginning of the universe and witnessed its creation by a mysterious giant blue hand, corrupting it with his presence to create the original Multiverse. That story concept by itself is relatively simple, but you can already see how this simple bit of backstory starts to lay roots through the bedrock that the entire DC Universe is built on.
There have been multiple Krona appearances in many major crossovers over the decades. Each time tends to correspond with some major shake-up of the whole notion of DC's Multiverse. He would go on to be a recurring villain of galaxy-spanning proportions. He was even revealed to be the final baddie of the huge Darkest Night crossover storyline.
What's most interesting about Krona, by my estimation, is that he's not a traditional comic character that has a specific visual or stylistic hook that makes him interesting. His niche is entirely maintained by his role in the larger continuity, and as a fan of continuity I find that fascinating.
Our Krona Story
There is a lot of content here. Krona has a relatively small number of appearances but the scope of his story is surreal. We're actually only using him for a very particular purpose, however, so almost all of his content is being left out of our story. We're not using Blackest Knight. We're not using any of the Crisis. We're not even using the Multiverse. Almost every part of his story is cut away. What we DO want to keep is the events of his original tale and the way they play out in relation to the Guardians of Oa.
Very specifically; Krona is one of the original Maltans that colonized Oa and became immortal. He then goes on to use Oan super-science to go back in time and view the creation of the Universe. Here's the core change we're making; his viewing of this event, rather than creating the multiverse, instead infuses the universe with entropy. This in effect also places the anti-oan flaw in the universe, which will fuel a number of Green Lantern villains in the future. This event is what actually inspires the Guardians to begin policing the universe.