So basiclly, when completing the quest That's What Friends Are For.. ur able to choose from a number of rewards, including Horn of the Argent Fury, If you look at the tooltip it says "Equip: Your melee and ranged attacks have a chance to chastise your enemy, dealing 1024 to 1536 Holy damage.".
HOWEVER! This can proc on ALL ranged and melee attacks, including non-hostile.
So basiclly, what u need to do is go to any capital and ask for a Lunar Festival Invitation, use the scroll to portal to Moonglade, Talk to Valadar Starsong and buy Elune's Candle for 10s. Please note that these are Unique so u can only carry one of these, But it has 88 charges.
So basiclly, eqip ur trinket and start spamming the Firework at enemy players and they will suffer from 1024 to 1536. And Guards Will NOT attack you!
Enjoy!
Please note that you have to complete the quest Crusader Forward Camp before you are able to recive the quest. Both quests requires you to be level 74 and can be found in Zul'Drak. If there is any item or NPC that you dont know where it can be found, try clicking the name and you will be sent to a wowhead page containg more information about the NPC/Item.
And also this is worth noting: