2200 BCE - The insectoid Tchk-Tchkii's life cycle of consumption and devastation lead the Guardians of Oa to erect a forcefield around their home planet, shutting them off from the rest of the galaxy.
18 years ago - The Tchk-Tchkii, after spending several millennia trapped within their planet, develop an anti-oan war-homunculus and uploads their entire hive mind, escaping their prison and setting out to destroy the Guardians of Oa. They murder dozens of Green Lanterns including Abin Sur. Hal Jordan is selected as his replacement and goes to Oa to train. When Legion arrives to kill the Guardians, Hal is able to absorb enough power from the battery to destroy the Legion armor. The protoplasmic remains of the Tchk-Tchkii are returned to their home planet.
I have a very vivid memory of reading the comic where Legion debuted. I was largely unfamiliar with Green Lantern before that comic, but the sheer scope of the sci-fi universe it depicted had me immediately hooked. While looking back, Legion might have just been a giant blunt instrument used to punch the hero into oblivion, he served a great purpose in that story and I still remember him fondly, even if I appear to be the only person doing it.
Legion's Comic History
This one should be pretty easy.
Legion has made what amounts to one appearance ever, which seems insane as he was the in-continuity explanation for Hal Jordan's predecessor's death for several decades. He was introduced in Green Lantern: Emerald Dawn, a limited series from 1989 that retold Hal's origins. It was written by Keith Giffen, who is often the man responsible for a lot of the space-opera feel of 80's and 90's comics.
Geoff Johns would retcon a lot of Emerald Dawn when he wrote Green Lantern Secret Origin in 2008, and of course the entire history of the Green Lantern Corps was massively rewritten in the post 52 era. The explanation of Abin Sur's death has been incorporated into a huge tapestry of prophecies and politics. For whatever reason, Legion has simply vanished into the ether of comic history.
Our Legion Story
If we're going to get rid of the expanded story of the Lantern Corps as written by Geoff Johns then there is a gap in our timeline that explains what actually happened to Abin Sur that lead to his death, a vital part of Hal Jordan becoming the Green Lantern of Earth.
The solution is obviously Legion, a character that was specifically built to do EXACTLY this. He's an ancient powerhouse bad guy specifically built to challenge (and kill) our hero, created three years before Superman met Doomsday and took this premise to it's logical endpoint.
Legion's single appearance happens in a book telling a existing character's ORIGIN, which means that he never actually appeared in ongoing continuity. He fills this very necessary role in a great way, and I for one love this character.