An effect modifier is a modular rule that changes the behaviour of other game content, like a buff/enchantment that makes a character stronger (or debuff/curse that makes them weaker), adds new gameplay abilities & behaviours, or changes how an existing rule works.
This tag was created in response to this Meta discussion.
Common routes to implementing modifiers include...
A fixed collection of stats that cover the variety of ways an effect can be altered (eg. health, damage, speed, moves, critical chance...). Each entity/effect has base values for the relevant stats, and modifiers add/subtract increments when added or removed. (Or the full collection is iterated on demand when the stat value is read)
A (prioritized) list of stacking alterations to apply, where each effect iterates through the list and applies each alteration before committing the final effect. This also gives the opportunity of modifiers to interrupt the iteration, to supersede later modifiers in the collection (eg. an "immunity" effect stopping an "on damage taken" effect from triggering)
Event systems, where modifiers subscribe to gameplay events to react to and modify, using the Observer Pattern.
Scripting languages, often in conjunction with the Bytecode Pattern, where each modifier can be a custom program running in a virtual machine to give maximum flexibility in the effects it can express in communication with the game's API.
Further Reading:
