The CriticalHit Event is fired whenever a player attacks another creature. It has a result which determines whether or not default crit behaviour occurs:

• allow: the attack is forced to be a critical
• deny: the attack is prevented from being a critical
• default: the attack uses default Vanilla behaviour to determine if it's a crit.

You can also adjust the damage modifier for the critical hit.

event.oldDamageModifier contains the original damage modifier of the event, while event.isVanillaCrit is a boolean value that determines whether or not this is actually already a critical hit.

You will need to cast the event in the function header as this class:
crafttweaker.event.CriticalHitEvent
You can, of course, also import the class before and use that name then.

CriticalHit Events implement the following interfaces and are able to call all of their methods/getters/setters as well:

• IPlayerEvent

The following information can be retrieved from the event:

ZenGetter	ZenSetter	Return Type
ZenGetter target	ZenSetter ​	Return Type IEntity
ZenGetter oldDamageModifier	ZenSetter ​	Return Type float
ZenGetter damageModifier	ZenSetter damageModifier	Return Type float
ZenGetter isVanillaCrit	ZenSetter ​	Return Type boolean

• event.deny() sets the result to deny.
• event.allow() sets the result to allow.
• event.default() sets the result to default.