Well it's tricky because they were from vastly different time periods, but here goes...
With Arthur and his knights returning from the Crusades, where they've been searching for the holy grail, conditions at home have worsened. Robin and his merry men are fomenting open rebellion across the land, with a special emphasis on Nottingham and its cruel Sheriff. But the Sheriff was lawfully appointed by Camelot so Arthur must ride to his aid.
Secretly, this plot was hatched by Arthur's estranged half-sister Morgan leFey, and her lover Prince John, the younger brother of the Richard I, King of France. They wish to subvert England for conquest by France, in the hopes that Richard (who is Arthur's staunch ally) will die in the Crusades, making John the King of England and France.
British folklore is full of tons of super creepy evil monsters, so this campaign will totally tap into that, too. Many of them already exist in D&D (ogres, sprites, hags, banshees, elves, revenants, green dragons...). You can also pull in mythical and historical figures from other periods. The English nobility must fend of vikings invading from Denmark. Robin Hood is allied with the pookas, pixies, etc. Maybe Cuchulain is traveling through England on his way to train with Scathach. And Merlin is just friggin' everywhere in British mythology (as a later addition to the tales).