With WorldEffects you can add potion effects to all players in a specific world.
So whenever a player enters the specific world the defined potion effects will be given to that player.
The plugin is fully configurable in the config.yml file.
/worldeffects <version | info | enable | disable>
worldeffects.worldeffects
Provides anccess to the /worldeffects command
A detailed description of the config file can be found here.
A list of potion effect IDs can be found here.
The source code of WorldEffects can be found here.
