Mutators

Mutators are a feature of Co-op Missions that were introduced in patch 3.3

Overview
Mutators are special conditions that bring additional challenge and diversity to existing Co-op Missions. Mutators can vary wildly, from having to fight a completely cloaked army, to dodging lava erupting from the ground. Mutators vary on a weekly basis. Each week, players have the chance to take on a Weekly Mutation where several Mutators are applied to a Co-op Mission. The mutation will be available for seven days with a break before the next one begins. Difficulty levels can still be selected, as can normal Co-op Missions, with the ability to toggle between the two modes.

A player must have reached level 5 with their co-op commander before they can test the mutators.

Bounties
If players complete the weekly mutation, they will be awarded a tiered XP "bounty" bonus based on the difficulty level selected. Completing a weekly mutation on a higher difficulty level will award the cumulative sum of all XP bonuses up to that level. For example, completing the weekly mutation on Hard will award the sum XP bonus of Casual + Normal + Hard.

Train of the Dead
''The infested terrans on Tarsonis are everywhere. They're in the ground. They're on the trains. Before long, they'll be in your base. And you may never see them coming.''

This mutation applies to "Oblivion Express." The following mutators can be applied:


 * Walking Infested: Every enemy commanders destroy will spawn infested terrans. The more life a unit has, the more infested will rise from it's corpse.


 * Outbreak: Continuous waves of the infested horde march against the commanders' bases.


 * Darkness: The fog of war is replaced with pure darkness and the mission’s objectives are hidden from the mini-map. Players won’t know where the trains are or where attacks are coming from.