How to make a weapon that spawns an entity when an entity is hit that attacks the entity that got hit by the weapon.

I'm pretty sure that there is a procedure block to spawn an entity and get its reference, or something like that. You could make a local entity variable, then set it to spawn a vanilla entity and get its reference, then check that it is not null, or check that it is a living entity (to avoid crashing the game). Then you can use the variable to refer to the newly-spawned vanilla entity instead of event/target entity, which will refer to the victim.

I think. I can't remember the precise name of the procedure block, and I haven't used it a lot, so I could be getting something wrong.