The EntityLivingSpawn Event is fired whenever an entity tries to join or leave a world.
It has one subclass, the EntityLivingExtendedSpawnEvent that also contains an IMobSpawnerBaseLogic reference.

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

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

• ILivingEvent

Die folgenden Informationen lassen sich von diesem Event (Ereignis) abrufen:

ZenGetter	Type
ZenGetter world	Type IWorld
ZenGetter x	Type float
ZenGetter y	Type float
ZenGetter z	Type float
ZenGetter ​	Type ​
ZenGetter spawner (Extended Only)	Type IMobSpawnerBaseLogic

The despawn event also offers three functions to change the event outcome:

ZenMethod	Beschreibung
ZenMethod allow	Beschreibung Forces the entity to (de)spawn
ZenMethod deny	Beschreibung Forces the entity not to (de)spawn
ZenMethod pass	Beschreibung Sets the event result to the default state