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

The following information can be retrieved from the event:

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

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

event.allow -> Forces the entity to (de)spawn

event.deny -> Forces the entity not to (de)spawn

event.pass -> Sets the event result to the default state

The following information can be retrieved/set during the event:

ZenGetter	ZenSetter	Type
ZenGetter entityLivingBase	ZenSetter ​	Type IEntityLivingBase