In the beginning of Watch Dogs 2 you can hear Sitara mentioning a “Secret Cow Level” over the radio. For the uninitiated ones -- the “Secret Cow Level” is a reference to Diablo II. It is a location that was renowned for a huge number of Hell Bovines (cows with polearms) that allowed for an easy level up.
You could also fight the Cow King as the boss of the “Secret Cow Level.” However, later on Blizzard changed the purpose of the location and it couldn’t be used for an easy grind anymore. Now, why would Sitara mention it in the Watch Dogs 2? This question still remains a mystery.