An IEntityDropFunction is called whenever the associated entity is killed. Leave it up to your imagination what you can do with this:

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import crafttweaker.entity.IEntityDropFunction;

The IEntityDropFunction is a function with the following parameters:

• IEntity entity → The entity that just died.
• IDamageSource dmgSourve → The source of the entitie's death.

The Function needs to return an IItemStack, or null.