I'm planning on creating a campaign setting that I'm calling the Abandoned Realm, partially inspired by games such as Elden Ring, Hollow Knight, and the Blasphemous games. One of the main problems I'm having, though, is something regarding incorporating a part of the world's lore into the world proper.

Basically, before the events of the campaign, two factions ruling over the Abandoned Realm went to war with each other. Neither side emerged victorious, wiping each other out and leaving the ruins of their castles and outposts littered across the map. A short while later, an event called the Severing occured, which broke the natural cycle of life and death in the realm. I wanted to have these two factions connected to the Severing in some way, but I'm struggling to figure out how to do it, and how to make ways for the party to glean this information from what little remains of that ancient war.

This is my first time creating a setting this big (nowhere near the size of the Lands Between, but still far larger than what I'm used to), so a lot of this is new to me. I'd love to hear your ideas on how to incorporate this story element into such a large world!

Make it up as you go. You have plenty of time. Two civilizations ruined each other before a major catastrophe hit the land. Nobody knows quite how, and exploring the ruins only hints at it. To truly know, one would have to have magics to travel time or spend a lifetime of adventure exploring ruins, artifacts, and other remnants to learn.

It's just something to sprinkle all about the place while your players figure out what their characters want to do in their present.

Our own history has several stories that we know happened but don't know the real details about. We get by fine - the characters will too!

Well, first, is it important that the Severing occurred after the war, rather than during it, or perhaps being part of the same incident which destroyed the factions? If it was a weapon gone wrong, people who know there was a powerful weapon but don't realize it's connected to the large-scale catastrophe might try to recreate it without realizing the full scope of the consequences.

Alternatively, and more compatible with the Severing being distinct from the war proper, perhaps it's left groups of undead from the factions, ghosts or whatever, who are still fighting, or playing at fighting, either because they don't realize the war is over or they are fully aware but can't let go of old grudges. This could either be a common thing or a unique phenomenon in one place as a sidequest where the players learn more about the backstory.

The first idea is probably more conducive to a slower-burning mystery, as the search for dangerous knowledge from the old war can be a recurring plot element even before you start to bleed in details about how or why these artifacts might've been used originally. By contrast, obviously the ghosts would probably know a lot more about the war they were fighting in life.

How far in the past are we talking? If it's only a couple of decades then you'll need to have the details much more locked in as your player's characters would have lived through it where as if it's centuries or even millenia ago you can leave it hazy, as Agile DM says there's plenty in our own world we only vaguely remember and every one gets along ok.

To tie the two events together I think BatJamags has the best idea, have the Severing as the event that ended the war, maybe as a super weapon that got out of control and destroyed both sides. You could leave conflicting accounts of whose weapon it was with the few survivors both blaming the other side for the destruction. You could even build up this uncertainty with more and more conflicting evidence when the truth is it was a third smaller faction, maybe even a single mage, who was caught in the middle willing to do anything to end the war for either a love of peace or revenge for their family being killed and ended up totally destroying both sides