Lactose Intolerant

To open a portal to the secret cow level you have to power up the 6 "runes" on the ground. To do this players must stand on top of them in Phase 4, get afflicted by [spell=296018] and then get dispelled. The [spell=295999] on the ground will power up the runes as you can see below.

Once all 6 runes have been powered up a portal will open in the middle of the room. Press the extra action button and you will be teleported to the secret cow level! From there you have a limited time to defeat 10 Twinklehoof Bovine.

This achievement is trackable so when the criteria has been met you can kill the boss and get the achievement.

Fun fact! The groups going for World First Glory of the Eternal Raider have used the [item=131724] and found six invisible npcs (turned into demons due to the toy's effect). This has helped a lot in discovering how the achievement works.

Spoiler

There is no cow level

Full Wowhead Guide Here: [guide=8780]

Have players position on the 6 red circles (there are 5 in a half circle in the western part of the platform and 1 in the east part of the platform) in phase 4 and dispel the Manic Dread fear while they are on top of the circles so that they drop the Caustic Delirium eyes on top of the red circles, when all 6 red circles have the Caustic Delirium eyes on them a portal to the secret cow level will spawn in the middle of the room and inside you need to kill the Twinklehoof Bovine quickly before the portal closes. Save Time Warp / Bloodlust / Heroism for the cows.

I am surprised no one have commented "Secret Cow lvl"