The Unquenchable Flame is a magical torch owned by King Arthur, said to be from the Burning Bush itself, that never extinguishes.
Before the Third Curse
Arthur and David collect the torch. ("Siege Perilous")
Snow, 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, including the Unquenchable Flame. The two of them bond on their journey, which is lit by the Unquenchable Flame, 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 undead knights who drag him underwater. Arthur rescues his new comrade, but the Crimson Crown is lost. ("Siege Perilous")