"Kanai's Stomping Grounds" is a version of the cow level made in tribute to a Diablo 3 team member who passed away.
Diablo 3's 2.3 patch brought with it an iteration of Diablo 2's Horadric cube attached to a rather heartwarming story. Dubbed "Kanai's Cube", the new item was named in honor of Kevin "Kanai" Griffith - a Blizzard artist who passed away in 2014. Well, it seems Kanai's cube has held a second secret dormant since 2.3's release this year: another secret cow level.
To access the level, you'll have to travel to the Immortal Throne in Act III's Ruins of Sescheron - where the cube is originally found. Up until now, the spirit of Kanai would simply tell players that "The time is not right", but during the month of March, he will open up a portal to "Kanai's Stomping Grounds".
The reasoning behind this is that March is the birth month of the real-life Kanai. A Blizzard blue poster confirmed that the portal would disappear at the end of the month.
Kanai's Stomping Grounds differs slighty from the other cow levels in the game: there is no Cow King or Queen, and it appears to lack the abundance of chests from the original level. However, the spirit of Kanai himself will help you slaughter cows, and even at Torment X he is a force to be reckoned with.
For the record, there are actually five additional "secret" levels in Diablo 3. The first, Whimsyshire, is accessed by assembling the Staff or Herding and taking it to the spirit of the Cow King in Act 1. The second, Development Hell, randomly spawns as one of the four crypts in Act 1's cemetery. The third, the original secret cow level, is a crypt-themed rift filled with cows and the Cow King, and has a chance of randomly appearing whenever starting a regular rift. The fourth, Greed's Lair, is accessed either by putting a puzzle ring into Kanai's Cube, or via a portal that has a small chance to appear after killing a goblin. Finally, the second cow level is accessed by putting the Bovine Bardiche into Kanai's cube.
This means that Kanai's Stomping Grounds is technically the third "cow level" in the game.