i found this by accident when i finished doing my World quests some weeks ago and entered a Mythic 10
i was still able to use the Special Action button to use the Spear on the Mythic+ Bosses (tested in EoA)
it does quite some damage (i don't remember the exact numbers) but it does somewhere around 2-5M Damage if i remember correctly
u can only use it once inside the Mythic+ ( can use it every time on a raid Boss aslong u go and get the buff again)
but with 5 players doing it in a Mythic+ or 20 in a raid it can make a massive difference in kill times!
u can find the Spear here :
(South Val'sharah)
The Spears spawn randomly here so look for a object that looks like this :
[Deepsteel Trident]
568035-deepsteel-trident-a-deepsteel-trident-stuck-in-the-sands-of-kaldelar.jpg