Forcing two wands that share the source of their cores to do battle can also cause a more potent form of Priori Incantatem. The tips of the two wands will connect, forming a thick golden "thread" of energy, and the two wands' masters fight a battle of wills. The loser's wand will regurgitate shadows of spells that it has cast in reverse order. This phenomenon occurred during the duel between Harry and Voldemort at the end of Goblet of Fire. Their simultaneous spells (Harry and Voldemort cast "Expelliarmus" and "Avada Kedavra" respectively) triggered the threads, and as Voldemort lost the battle of wills, his wand regurgitated, in reverse order, shades of the spells he had cast with it – screams of pain from torturing various victims, echoes of the people his wand had murdered: Cedric Diggory; Frank Bryce; Bertha Jorkins; as well as Harry's parents. Dumbledore later revealed to Harry that his and Voldemort's wands both shared a tail feather given by Dumbledore's pet phoenix, Fawkes. Harry was previously informed by Mr Ollivander that the holly wand that "chose" Harry was the "brother" of the yew wand that gave him the lightning-shaped scar on his forehead, although the significance of this was not discussed at that time.