Bukkit

Here is a little example to understand how the config is built up:

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effects:
    my_rpg_world:
      zombie:
        nausea:
          multiplier: 1
          chance: 100
          duration: 10
        lifesteal:
          chance: 45
          infmsg: The zombie has eaten your flesh, to replenish his own health!
      skeleton:
        blind:
          multiplier: 1
          duration: 4
          chance: 60
          infmsg: The skeleton has tampered with your vision!
          curemsg: Your vision has stablized.
      spider:
        slow:
          multiplier: 1
          duration: 2
          chance: 90
          infmsg: You are tangled in web!
          curemsg: The spider's web has fallen

In this case, in the world with the name my-rpgworld:

• a zombie would give you the nausea effect with a multiplier of 1, a chance of 100% and a duration of 10,as well as lifesteal with a chance of 45% and a message when the effect happens
  
• a skeleton would give you the bilnd effect with a multiplier of 1, a chance of 60%, a duration of 4, a message when the effect occurs and when it is "cured"
• a spider would give you the slow effect with a multiplier of 1, a chance of 90%, a duration of 2 together with a cure message and an infected message
  

  The world

  In the config you need to set up the effects for each world seperately. That includes world_nether and world_the_end too (if you want).

  The mobs

• multiplier: Strength of the effect
• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

• multiplier: Strength of the effect
• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

  The slow effect

  Name in config: slow
  
  Available options:

• multiplier: Strength of the effect
• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

  The blind effect

  Name in config: blind
  
  Available options:

• multiplier: Strength of the effect
• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

  The poison effect

  Name in config: poison
  
  Available options:

• multiplier: Strength of the effect
• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

  The burn effect

  Name in config: burn
  
  Available options:

• duration: Duration of the effect
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got infected
• curemsg: Optional message, sent when target got cured

  The lifesteal effect

  Name in config: lifesteal
  
  Available options:

• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when target got life from the attack

  The critical hit effect

  Name in config: crit
  
  Available options:

• damage: Sets the total damage for the hit.
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when enemy lands a crit

• damage: Sets the additional damage for the hit.
• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when enemy lands a crit

• chance: Percentual chance if the player get the effect or not
• infmsg: Optional message, sent when enemy lands a crit