[Astromancer]

Originally Posted by Sydney

long story short, I'm thinking of transporting them to another world, one where Mordred was successful in usurping Arthur's throne, though it's rumored that Arthur is yet alive somewhere...

One good trick on the true king might be to remember that Mordred was a virtuous knight until he was told that he was doomed to kill his father. The same "Holy Man" told Lancelot, at the same time, that his adultery would destroy Camelot. What if the "Holy Man" was in fact an evil wizard? What if Modred is enspelled? Perhaps Mordred is the true heir of Arthur who needs to be freed.

It would be a dark Britain, one where Mordred rules with an iron fist and his mother, Morganna works dark magics to help keep him on the throne,

Mordred's mother was Morgwase. Morgain's children were Uwain and, hold on to your hat, Galahad! Morgain was the Elain of Castle Corbek in many older versions of the story. It is Morgain who carries the Grail in Castle Corbek. Galahad's mother is Elain of Corbek.

That will surprise the PCs. Morgain works for her Gods and perhaps cuts a deal with Mary. Morgain, perhaps with Sir Kay as an ally (it makes sense if you know the Medieval Romances) could test the worthiness of the PCs, then ally with them to free Mordred.

However, I'm drawing short of a decent enough adventure...perhaps the knight who was attacked was transporting something of import and it was stolen, requiring the PCs to bust into the castle of a knight of the Round Table to retrieve said item...But, what to make the maguffin?

Balin's spear caused the disaster that the Holy Grail cured. As finding the Grail broke the Enchantments on Britain and freed the land from the curse of the Wasteland, perhaps those enslaving Mordred to Darkness want the spear of Balin to eternally curse Britain.