Why was there a "beam war" between Harry and Voldemort in their final battle?

The only canon explanation we have for that phenomenon in-universe is Priori Incantatem, which can only happen if the combatants are using wands whose cores came from the same source, and in this battle, Voldy is not using his wand, he's using the Elder Wand.

For that matter, how did a disarming spell rebound the Avada Kadavra back at Voldemort? I get that the Elder Wand wouldn't kill its real master, but as Moody states in the 4th book: the Killing Curse is unblockable.



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