When you think of stories about King Arthur, a few things come to mind: the Knights of the Round Table, chivalry, and the adulterous Queen Guinevere cheating on Arthur with his best friend, Sir Lancelot. But there's a lot more to Arthurian legends than a circular table and one scandalous affair.
The stories about the Knights of the Round Table include the lecherous exploits of Arthur's favorite nephew, Sir Gawain, the humble, kitchen-servant origins of his brother, Sir Gareth, Guinevere's family and food troubles, hunting magical enemies (ranging from warriors to boars), and so much more. Oh, and then there are Arthur's own scandals with his sibling and his sorceress sister's 99 problems. Arthurian Legend is definitely anything but the dull stuff of Medieval legend.
It's Christmas at King Arthur's court and the royals are all gettin' down around the Round Table as they do when, suddenly, a giant knight with green skin busts into the Great Hall. For totally inexplicable reasons, he offers up the chance for one knight to give him an ax-blow to the neck in exchange for the same treatment being given to the knight one year and a day later. Arthur's nephew Gawain accepts the challenge, removing the head of the Green Knight, who then promises to meet Gawain in a year's time.
In the year that follows, Gawain sets off to find the Knight at his Green Chapel, hoping to reach some sort of... compromise. On the way, he gets involved in all kinds of trials and tribulations and ends up playing a kissing game with a lovely woman whose husband he has promised to give any gifts he receives from her. Gawain doesn't do this, however, and he keeps a magical girdle that she gives him for his quest. Whatever, Gawain has bigger fish to fry than husband-wife dishonesty.
When he finally makes it to the Green Knight, Gawain flinches at the prospect of having his head cut off, and the Knight only breaks the skin - he doesn't chop all the way through. Turns out, though, he's the husband of the lady Gawain kissed; the almost-beheading was punishment for not fulfilling his bargain of giving him the girdle. Embarrassed, Gawain wears the girdle forevermore as a sign of his failings and desire to seek honor.
In his origin story, Arthur is born as the result of deception, magic, and rape. His father, Uther, is High King of the Britons but doesn't have a wife, having fallen deeply in love with the spouse of one of his vassals. She is the lovely Igraine, Duchess of Cornwall, reputed to be the most beautiful woman in all of the land. At court, when her husband Gorlois sees Uther eyeing up Igraine, he takes her home, and Uther gets furious.
Uther then rises up against Cornwall, besieging Gorlois in one castle, while Igraine gets shut up for safety in the seaside fortress of Tintagel. However, regardless of the conflict he's involved in, Uther insists on making Igraine his own, so he convinces the magician-prophet Merlin to help him. Merlin transforms Uther into Gorlois so he can enter Tintagel. Thinking that he's her husband, Igraine has intercourse with Uther, with no one the wiser. In that one deceptive night, Arthur is conceived.
While Uther was assaulting Igraine, his armies were engaging Gorlois's, and they slew Uther's rival. In the morning, Uther goes out again, assumes his regular guise, and then weds newly widowed Igraine, by then pregnant with his child.
After marrying Uther, Igraine has a few problems - namely, her two daughters by Gorlois, Morgause and Morgan (later nicknamed "le Fay" for her otherworldly abilities). The exact number and names of these girls vary depending on the storyteller, but these are the two most recognized half-sisters of Arthur. In truth, they never really know their little bro since he gets raised anonymously in the household of a loyal knight.
So, when Arthur meets his half-sister Morgause (or Morgan, depending on the story), he doesn't realize who she is and has an affair with her. As Thomas Malory, one of the most famous Arthurian chroniclers, writes, "He begat upon her Mordred, and she was his sister, on his mother's side, Igraine." This incestuous tale takes an even darker turn when Arthur learns that Morgause had his baby, and he pulls a King Herod, ordering all babies born on May Day in Britain to be slain. Unfortunately for him, his son-nephew, Mordred, survives - oh, and his subjects end up really hating him for slaying a lot of innocent babies.
As an adult, Mordred, seduces his aunt/stepmother, Guinevere. This is seen as super-shameful on Guinevere's part; she'd already betrayed Arthur with Lancelot, his friend, before going off with is own son.
When a lady wearing a sword shows up at Arthur's court, everyone is stunned. As with many of the craziest stories of Arthurian legend, she also issues a challenge upon her arrival - let the knight who is without blemish draw the sword from its scabbard. Many knights try and fail at the task. That is, until a poor, shabbily dressed knight named Balyn amazes everyone with his success. After he draws the sword, however, he doesn't want to give it back. The Lady warns him, though, that if he keeps it, he'll slay the person he loves. Yikes. But then the Lady of the Lake shows up, demanding either the swordswoman's demise or Balin's. Quick-thinking Balyn solves the problem by slaying the swordswoman. Later banished, Balyn winds up wounding the eventual Fisher King in a fight (that's an important moment for the Grail Quest, FYI).
But his most awful moment really comes when he decides to challenge a stranger knight - actually his brother Balan - who's wearing armor he's never seen before. The two knights slay one another in combat, but not before each realizes the one he's slain is his sibling.