As the legend goes, King Arthur is the son of Uther Pendragon and Igraine. He was taken from his father by Merlin who raised him along with Kay. Uther dies and the kingdom was without a king. So, a tournament was held and Arthur, acting as Kay's squire, forgets Kay's sword. He runs to retrieve it, but comes across The Sword in the Stone instead. Not knowing the legend behind the sword, Arthur grabs it and runs back to Kay. Kay notices that it is not his sword, but the famed Excalibur Sword. After much ado, Arthur is made King.
After awhile, Arthur runs into some trouble with his kingdom, and decides to form the Knights of the Round Table to help fight off the Saxons. They succeed but the kingdom falls into disarray, with demoralized citizens and quarrelling knights. So, Merlin tells Arthur to send the Knights out to find the Holy Grail, which could save Camelot. They never bring the Grail back to Camelot, and they apparently all die or go mad...but the Knights Templar continued their Quest for the Holy Grail. Were the Knights of the Round Table and the Knights Templar the same group of knights? If so, this is one of the major places that legend lost the details.
By taking a look at physical evidence in addition to historical evidence, we can get a better idea of the situation. During his life, Uther Pendragon possessed a sword that was famous in battle. The main design on the pommel was a stylized cross. Not the Christian Cross, but a cross of equal lengths like a plus sign. This design can also be found on King Arthur's sword Excalibur and the Templar Knights Sword. Lancelot's dagger also carries this design, though it is much more discreetly shown.
Is it a coincidence that this design continues to show up on various swords, shields, and helmets throughout medieval history? Is there a possibility that the Knights of the Round Table fostered the Knights Templar whose legend continues on for hundreds of years?