"Leads to a place that does not exist. The Burning Hells are not responsible for events that transpire there. If you claim to have been to this place, you will be called a liar. Void where prohibited."— Portal flavor text(src)
The Not The Cow Level is a secret level in Diablo III, first implemented for the game's third anniversary.
Originally, this event was only available for a limited time: from dawn on May 15 2015 to dusk on May 21 2015, there was a chance for Herald to the Queen to spawn, which, when killed, would leave an opened portal to the secret level.
It can only be opened once per game, and once inside, the entry portal can be used to travel back. The portal stays open for the rest of the game, even after the zone is fully cleared.
As of patch 2.3.0, one may use Kanai's Cube and put a Bovine Bardiche in it to reveal a hidden transmute option (marked ???). Performing this transmutation will destroy the weapon, but open a portal to NTCL.
The zone is always based on the Fields of Misery, but it can be opened from any map on any difficulty level in Adventure mode.
A full set of one of each of the six base shrines are found, along with Healing Wells. Unique T-Bone Steak health pick-ups will drop from killed enemies along with normal Health Globes (similar to the cupcakes in Whimsyshire).
The boss of the realm is The Cow Queen and the ghost of The Cow King is also found, uttering (or, in his case, uddering) various bovine puns, triggering the final event "Tipping Point" requiring 3 waves of bovine herds to be eliminated to spawn 4 Radiant Chests.