Chaos (AKA Nightmare in CF NA, Gargoyle in CF ID) is one of the Challenge Mode boss mutant, appearing in Round 30 in Secret Valley and Venice maps. He also makes a minor appear in Valkyrie Lab and he's completely harmless in this time.
- Beacon: The beacon on its chest will temporarily blind you. (lasts for 3 seconds)
- Slash: Chaos will attempt to slash whoever its target is. It's usually easy to dodge. (deals 35 damage)
- Charge: This attack is really hard, nearly impossible to dodge, dealing 50 HP. It will fly up in the air (25 HP) and charge at whoever it's attacking (25 HP).
Secret Valley (Biohazard Lab)
This map makes it easier to defeat it than Venice, due to the flatter and more open area where you encounter it. The barricades are really useful to trap Chaos and make its attacks easier to dodge, sometimes it won't be able to get you at all. You should keep every weapon (Rifles or MGs) found within the last area and bring them near the barricades where the person attempting to lure it will be. Because if the person runs out of ammo/guns, the boss will simply move on and seek somebody else shooting at it and it might be hard to get it stuck again. It is recommended that other players do the luring tactics as well, especially because of the scorchers.
This map is a mess when it comes to encountering Chaos. It seems that it wasn't made for it, simply by looking at it going through the bridges. It is possible to apply the same tactics. You lure the zombie hordes under the snipers spot. You can also make Chaos stuck if you make it come near the sewers and jump in the water once it does one of its attacks and you might get it stuck for a while if not until the end of the battle. Just don't forget to keep any weapons dropped by dead players as well.
- It is still possible to melee-attack it, but it's really time-consuming and you would need to be on one of the barricades, so it is not recommended.
- Turn away when it's about to blind you, it's effective.
- Try to make it stuck somewhere and you can kill it pretty fast.
- Whoever lures the boss should be the first one to shoot it.
- One person has to do the luring tactics.
Beating it 5 times will get you a Chaos (Nightmare) Boss Crate. You could get various Challenge Mode exclusive items, including boss related items:
- Chaos Goggle: Mask representing Chaos's face.
- Chaos Wing: A small backpack with a pair of Chaos's wings.
- Chaos Hook (Nightmare Blade): A slower Mutation Spike representing Chaos's claws.