I think you should put Excalibur on the duke of Cornwall in the Charlemagne start. My reasoning is because of The Mabinogion gives it to them.

Then they heard Cadwr Earl of Cornwall being summoned, and saw him rise with Arthur's sword in his hand, with a design of two chimeras on the golden hilt; when the sword was unsheathed what was seen from the mouths of the two chimeras was like two flames of fire, so dreadful that it was not easy for anyone to look. At that the host settled and the commotion subsided, and the earl returned to his tent.

— From The Mabinogion

You could maybe do a durandal - the sword of count Roland from "chanson de geste"

@Dolarek read the description - Durandal belongs to Roland already.

https://en.wikipedia.org/wiki/Holy_Lance#Antioch

@The ♥♥♥♥♥♥♥♥♥♥♥

Already in vanilla :-)

The Tizona and the Colada

Already in the mod

https://en.wikipedia.org/wiki/Stone_of_Scone

The Book of Kells.

The Ark of the Covenant

Once you finish your current to-do list some Hellenic artifacts that can also be accessed by the Greek and Roman culture groups would be cool

Since you already did Cid's swords:
https://en.wikipedia.org/wiki/Cross_of_the_Angels
https://en.wikipedia.org/wiki/Victory_Cross
https://en.wikipedia.org/wiki/Agate_Casket_of_Oviedo
https://en.wikipedia.org/wiki/Sudarium_of_Oviedo

Can you add a event chain to get a damascene steel sword?
Look it up, they are awesome. Here's how I see the event chain going.
1: You are reading in your library, where, in a dusty old book about swords, you discover information describing damascene steel, some kind of super-metal that can cut a falling piece of silk cloth in two, effortlessly, and cleave through chainmail as if it were butter.
2: You call a meeting of your council (like in the immortal questline) and decide to travel to damascus personally, to find a smith who can work damascene steel, and make it into a sword. There's one or two events for your journey there, like you travelling to venice and organizing travel there
3: You arrive at damascus, and go to the market, asking about damascene steel, and one blacksmith overhears you, and directs you to a secret guild of blacksmiths who can forge damascene steel blades. However, as you go to the building where they reside, you are ambushed in the alley by several soldiers. If you are defeated by them, you wake up in the port and return home. If you defeat them, they reveal that the ambush was a test of your true abilities, as only a true warrior can wield damascene steel.
4: They blindfold you, and bring you to a secret building (the other was just a lie from the smith to get you to take part in the test), where you meet the guild. Since you passed the test, they forge something for you. it can be A: A Bastard Sword. B: A greatsword. C: A dagger. or D, A warhammer.

The bastard sword grants you +2 diplomacy and +4 duel skill. The greatsword grants +2 martial, and +4 DS. The dagger grants +2 Intrigue, and +4 DS. The warhammer grants +2 Stewardship, and +4 DS.

All of them should also give you increased prestige gain, for obvious reasons.

However, you can forgo all of those options in exchange for joining their society, which doesn't give you a sword, but a book that explains how to forge damascee steel which grants you +6 Learning. Also, if you choose this option, it triggers a event sometimes where a prospective buyer for your damascene steel comes to your keep (a character in-game) and you can make them a sword, in exchange for 400 gold. It also gives them a damascene steel sword as a artifact.

If you actually want to do this event chain, I can help write the events themselves, you can get event pictures and actualy do the modding work, since I am useless at modding.

Also, the censored sword name is another, meaner word for illegitimate child.

Heirloom Seal of the Realm of China.It should add five diplomacy,prestige and piety.Historically,it was lost and some suspect it was taken by the Khitans who are on the map.It should be useable by anyone.