I love when developers sneak secrets into their games without any hints whatsoever. Path of Exile 2 hid an entire subclass in its previous season that you could only get by fighting a unique boss on a specific class. People had to figure this out by word of mouth when someone accidentally pulled it off.
The Diablo series is known for the secret cow levels that Blizzard playfully refuses to acknowledge despite revisiting the reference in several games. Diablo 4, however, doesn't have one yet, no matter how hard people are still trying to find it with every new update.
