As you might know, there is a special event in World of Warcraft that Blizzard implemented for Diablo's 20th anniversary. We wrote an article about this event, so there won't be much detail on the event itself, but more tips on what is needed to get this event completed and its special Feat of Strength. We have done some digging and our own personal research to bring to you some information and the best way to get this achievement.
Your first step is to locate a treasure goblin. You can locate them at the end of Legion dungeons (on any difficulty), or around the world in Broken Isles or the Underbelly in Dalaran. Please remember that it is wise to pay a guard 5g so you won't be attacked while in the underbelly. Once you are down in the underbelly, you might have to wait around for the Treasure Goblin, but it's well worth the short wait. After killing the Treasure Goblin (which must be done before he opens his escape portal and leaves), a portal to the secret cow level opens up.
Enter this portal, you will be then taken into a version of (what looks like) somewhere in Duskwood. You may engage the Cow King, though there are cows there that are scaled with your level. The Cow King drops the Twelve-String Guitar. This a toy can be learned, and you obtain the Feat of Strength after learning this. This toy plays a melody from Tristram, and it appears to be a guaranteed item drop.
There is not much more to the Diablo Event in World of Warcraft. You can obtain loot once per day per character, similar to holiday bosses. There are other loots the Cow King drops, as well as Goblins. The most important one is that twelve-string guitar for those who wish to achieve the Feat of Strength. The cow level, should you get in, is quite fun and short. It does not take long to kill the Cow King. The best part about the Cow King & the Secret Cow Level? You do not have to be level 110 to get into the cow level. Though a minimum level has not been stated, level 101s have been able to enter this secret level.
You can obtain scrolls of town portals which share a cooldown with your Hearthstone. You may even get a book that allows you to combine 10 of the scrolls in a tome. There are also other items that increase agility and mana as long as you have them in your bags. You do not need to equip them or "use" them, Just simply have them sitting in your bags.