Phantom Knights are the ghostly guards that live in the waters that surround Brocéliande, attacking anyone that dare enter the area.
Before the Third Curse
The Phantom Knights attack Prince Charming. ("Siege Perilous")
In Merlin's tower, Mary Margaret, Emma, Regina and Belle work to try and find a way to free the Sorcerer from the tree he's trapped in; it is soon decided that they need a way to communicate with him in order to learn more about his condition, and to do that they're going to need a certain magical toadstool known as the Crimson Crown, found in the Forest of Eternal Night. David offers to find it for them, and Arthur offers to go with him, first stopping by the Round Table to collect a chest of valuable items to help them on their way. While there, David is told of Siege Perilous, a seat at the Round Table reserved for the most pure-hearted of knights, formerly taken up by the now deceased Lancelot. The two of them bond on their quest over things such as their wives and their similar humble beginnings, before making it to the toadstool... which is across a bog, and the only way over it is a weak bridge. David carefully goes over it, suspecting that it will only take one, and successfully gets the toadstool; however, he is then attacked by the phantom knights who drag him underwater. Arthur rescues his new comrade, but the Crimson Crown is lost. Due to the valor expressed by David on this quest, Arthur knights him into the Knights of the Round Table, allowing him to sit down on siege perilous. David is honored, and his friends and family are extremely proud of him. However, Arthur secretly meets with Guinevere afterwards and reveals he stole the toadstool off of David, for he doesn't want them using it. ("Siege Perilous")