When the Dungeon alternate upgrades were shown to the community a few weeks ago, what people found the most intriguing were perhaps not their look but their special abilities, including the mysterious invisibility. Today, Acheval provided a detailed description of four Dungeon abilities:

"When it is activated, the unit becomes completely invisible for the enemy until it performs an attack or takes a position in a tile next to an enemy unit. The stack attacked by an invisible stack can not retaliate. An invisible unit can not be the target of any target-specific effect, nor do mass spells work on it (but the area-effect spells and area effects do). (...) Unit movement routes can be plotted through the tile occupied by an invisible unit (...). When the movement is performed, if the invisible unit blocks the way and there is no other way, the moving unit stops right before it and performs a melee attack. For the player/AI who owns it, the invisible unit shows on the screen as semi-transparent. The ability works once per combat. The invisibility effect’s duration is 3 rounds."

All four abilities have been added to the Creature Abilities page. Also, I believe I have found an exclusive screenshot of the invisibility ability in action. Wow.