Go to Vivian at the Lake near Owain
Ask Vivian excalibur or camlann
She then tells you to go to Avalon and ask the King Avallach about Camlann
Go to Avalon, (Altar not far from bishop you go to be knighted by Aurther) Ask king camlann
Ask king portal
He will tell you that you need 3 things, a tear of a resentful son (Mordred), A pagan earring (Malgven), and scars of leadership (Wonjuru) to give to Merlin to open a portal.
Gather those things. (Need a group of at least two for Mordred)
Go to Merlin (Above ground version), then one at a time put item brazier. NOTE: Do not rent or do before reboot. Recommend gathering the items THEN putting them in one after the next at the same time.
Ask Merlin Portal
Go to the bodies of Arthur exam corpse, you then pull out Excalibur, the king's sword
Go back to Vivian and give her the sword.
NOTE: IF you wish to keep the sword you can, and it appears that you could simply repeat the portal steps to have the sword and have the whois. Needs to be tested fully to verify.