This post describes Mutators in Unreal Tournament — specifically Unreal Tournament 3, although generally applicable to other version of Unreal Tournament (and other games based on the Unreal engine).

Engine.Mutator

The Engine.Mutator class is the root class for all Mutators.

Like most classes in Unrealscript, Engine.Mutator provides a set of class functions.

To create a Mutator, extend the UTMutator class, and call one or more of the UTMutator functions (more on this elsewhere).

List of UTMutator functions:

• AddMutator
• AlwaysKeep
• CheckRelevance
• CheckReplacement
• GetDefaultWeapon
• GetInventoryClass
• GetInventoryClassOverride
• HandleKillMessage
• IsRelevant
• ModifyLogin
• ModifyPlayer
• MyDefaultWeapon
• PreBeginPlay
• ReplaceWith
• ReplaceWithClass

Source: http://greatemerald.eu/uncodex/UT3/utgame/utmutator.html

Caution – do not confuse Engine.Mutator with UTGame.UTMutator.

UTGame.UTMutator

TO DO – describe