Here's how I would do it. Each named entity (creature, event or named item) would have a status with each civ. This would be based on a number of factors, such as how great an impact it had with that civ, how long ago it happened, and whether or not the entity was associated with anonther significant entity. These statuses would be, with real world examples of a creature, object, and event:
Diety: Currently a diety, impersonating one, or associating with one. (Allah, The True Cross, The destruction of Sodom)
Mythological: Entities of great fame that are almost universally known but there is virtually nothing known about facts, often associated with deities. (Athena ,Gungnir, and The Founding of Rome.)
Legendary: These have strong (or at least plausible) historical basis, but are often sensationalised, mixed with half-truths, and mix in elements of other stories. (Roland the paladin, Caliburn, The Battle of Thermopylae)
History: The basic facts are know, although each civ has a different slant on it. (Stalin, Bocks Car, the Battle of Britain)
Forgotten: Nobody but the most pedantic scholars know of this. (?,?,?)
For History-level entities, the game will treat them exactly as they do know, with a little slanting for the civ. For example, the elf siege repelled by dwarves would be read by the elves as "Brave crusaders perish trying to stop the desecration of the forests) whil the dwarves would rememberr it as "Pointy eared freaks stumbled into our traps".
Legendary events might have the numbers changed, so that the 47-goblin vs. 15 dwarves becomes 5000 gobs against ten dwarves, and might have some other entities involved that actually were not involved.
Mythological entities would have the events themselves greatly distorted, with much being incorrect.
The main problem with this system would be filesize, as most entities will need as many copies of themselves as there are civs (To store "their" version of things.