This is a neat little trick I found awhile back while using my [Thunderfury, Blessed Blade of the Windseeker] in an ICC25 run. When the Val'Kyr Herald in the Upper Spire of ICC cast Severed Essence. My character wielding thunderfury was hit, and my clone spawned with thunderfury. At first I thought this was simply cosmetic, but too my delight when I started getting hit, I got hit with the thunderfury debuff. Spreading cyclones throughout the raid and slowing people's attack speeds.
Why you should care: It's always funny when someone looks and their debuffs and is dumbfounded when they see the Thunderfury debuff, it's been the basis for quite a few good laughs and it also looks pretty having your raid consumed by cyclones :P
I haven't tested this yet with other weapons with proc spells or abilities, but I assume it would work similarly for them all. As far as npcs that this works on, I can confirm it for the Val'Kyr Heralds and the Frostsworn General in Halls of Reflection.