Nightmare Mode is the highest of all difficulties, overriding the Squad Rating based difficulties when activated. Nightmare can only be activated early in the game with all players being atleast 500 exp or greater.
Because Nightmare is an extreme test of skill, several major gameplay features are added in addition to increasing enemy spawns and health.
In nightmare any automatic reload is double length. Because of this, it is essential to always manually reload. This is extremely important on weapons with long reload times like the XM814 Heavy Machine Gun, Barrett M112, and Stinger MK-3.
On nightmare many common enemies gain the ability charge. Charge works much like Zealot Charge in melee games. Enemies that reach a certain distance to players gain a burst of movespeed, allowing them to quickly close the distance and attack. This makes kiting more difficult and forces an extra level of concentration. Enemies that gain charge are as follows.
- Brain Bugs
- Brain Bug Queen
- Some Bosses
On Nightmare all five Infestors gain frenzy. Infestor frenzy increases their movespeed by 1% for each hit they incur, up to a maximum stack of 30. This includes knives and hits from skills. Because of this, and in combination with the speed increase from Burrow, Infestors can become very fast and quickly escape players grasps. They will lay additional eggs on Nightmare, and can become problematic if not properly dealt with. Stuns become more important against infestors, and vision granting skills are more necessary to follow them as they quickly move while underground.