This enchantment was cast by Merlin on Lancelot's spear. The spell he chanted was: "Bregdan anweald gafeluc!" He used it to allow Lancelot to kill the Griffin, a creature of magic that could not be killed by any mortal weapon (Lancelot).
When Kilgharrah attacked Camelot Merlin used this enchantment once more ("Flieh! Gár!") making a spear fly and hitting the dragon with it, but it didn't work on Kilgharrah for he was immune to Merlin's magic.
Merlin learnt this spell in Gaius' grimoire and, when he first tried it, it took him several tries to make it work, for it was the most powerful spell he had cast so far. When he tried it again against the Questing Beast he managed to cast it at the first try, demonstrating that his powers had grown.