Okay, here we go...
It's about the legend of King Arthur Pendragon, there are however various versions of the story but it's set during the Medieval Europe (Great Britain if I remember correctly).
The one I read started with the death of his father, Uther Pendragon (he did not grow up with him and people don't know that Uther had a son; can't remember the reason). After Uther's death, there was no heir to the throne, and there was this legend that whoever pulls the legendary sword Excalibur out of a rock/stone will be the next ruler, and surprise Arthur was the one who successfully and easily did it. Although other versions say he got it from the lady of the lake. It's followed by fighting those who opposed him ruling Britain with the help of Merlin the magician which became his mentor and adviser. Arthur winning wars anadviser. Later on he would make Camelot, the center of his court where he'll rule and live with Guinevere, his wife (there's a whole story about this, she was also a princess of another country...) whom he sincerely loved. He would also form the knights of the round table which would include Lancelot (each knight has their own story or adventure too). Plus there is also this quest about finding the holy grail. A lot more will happen TBH and I've read this around 10 years ago TuT so this is what I can remember.
Focusung on the two, Guinevere and Lancelot in this legend had an affair and most if not all of the versions identify their affair as what started Arthur's downfall.
If you want to know more about it, I recommend reading The Once and future king by T.H. White, although there are really A LOT of books about it. (⌒▽⌒)
Lancelot and Guinevere's story was really tragic, but loved Arthur more, so they got what they deserved (in the Arthurian legend). It's fun reading this new twist in that classic story tho. ヾ(❀╹◡╹)ノ~