Obtaining this is probably a lessening chance per day. (Success, success, failure, failure, success).
Haven't had much success in finding the "difficult to avoid in its native environment" feature underground:
A pale shape pulls itself from the ground and bites You for 12 damage, but your difficulty isolating a target lets them avoid all of the damage.
The bats swirl in the air, making it hard to pick any individual out among all the wings.
A pale shape pulls itself from the ground and bites You for 26 damage, but the stone absorbs all of the damage.
The serpent melts into the stone.
But its native environment might be underwater or cavern. It's not easy to find someone who can evade etheric in those locations though… (just ocean king or flooded sewers pvp for underwater, and no one at all for caverns). Or some monsters from items, maybe.
(I've reported the "You" in place of "opponent" typo.)
No special caverns text (same as other underground).
No effect on the final mining encounter: save the miners, stay and watch. Hey, I'll try anything to rescue those poor guys.
In the beginning, men were mortal.