Minecraft wiki entry for JSON_format, associated text:  conditions:
 entity: The entity that was killed. May be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 killing_blow: The type of damage that killed an entity. Missing corresponding list of predicates for the direct entity.
Tags common to all damage types[show]
 player: The player that would get the advancement. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]