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Crown adds you damage and hp equal to 999999999. So its like having the boss curse of retribution active till the end of the battle.
Anyways you can still be cced (crowd controled) with stuns etc and unable to act or if your hit is very low you may miss all the time.
Also another way to cancel crown is if you fight another player with crown or if someone else attack you and suicide so he can protect the main person that is protecting them.