Edit* phone messed up. How about just a large thrall, just over guardian size, who is completely invisible. There is a relic that is used like a gun, must aim down sights to use to spot the boss. You must then communicate to the others in your team where he is. Shooting the relic will make the thrall appear like a camo hunter for two seconds and must be recharged to be used again for 12 seconds but while aiming down sights you can always see him if lined up properly. Small, tight quarters. Maybe the size of a gladiator arena (on foot, not the chariot race type). The boss is fast, and if hit 3 times you will die. No health recovery. Maybe relic is just futuristic flashlight and the arena is almost pitch black. Would make for an awesome, terrifying experience. I imagine the predator 2 danny glover scene in the meat packing factory (think thats the right predator... And the right actor) [spoiler]this is off the top of my head and nowhere close to polished so feel free to add, change, criticize whatever you think would make it a better encounter[/spoiler]