‘No one likes a story that starts off, “Millions of years ago shit happened that no one cares about, no one remembers but it is very important.”‘
– Luke Crane
These aren’t meant to be a timeline or a history but a way to take a dungeon and inject it with historical radiation that will make it stranger and more rich. Don’t take these eras and wars as gospel but use them as inspiration, adding color to a battered dagger, tying the treasure to some olden struggle that still plays out in the world and echoes across your maps. They didn’t happen in a neat order but bled across one another in confusing ways that confound historians to this day.
Let your giant ants uncover the lost shrine of an Old Power. Have the mad vampire duke build his kingdom on the foundations of a dwarven fortress meant to keep a watchful eye on an elven forest. The ruined wizard’s tower holds relics from the Wizarding War and was a barracks for alien soldiers who once invaded this world before their empire abandoned them.
These aren’t meant to make the game, only spice it, add a bit to the dungeon stew.
Githyanki Diaspora 
I like tying the historical radiation into things inherently important and interesting to the players. For example, I ran some D&D with _dwarf-hating_ elves dungeoneering into a ruined dwarven holdfast infested with kobolds.
They discovered an ancient elven gravesite in the holdfast, the remains of a force which broke ranks from the elven host to stand with the dwarves rather than abandon them. The interaction later with a dwarven ghost went from hatred to something else when he asked if they were dwarf-friends like those honered, buried elves.
Doesn’t have to tie into a major part of the group dynamic, but I like things that catch the _player’s_ interest.