This script will allow your Attacks to deal increased damage once the hero reaches a critical HP level. Sounds familiar? Possible, because this is a recreation of FFX-2 accessory:
You can use one of the following note tags in the note box of Items, Skills, Weapons, Armours and States:
Simply use one of these and have an actor lose a lot of HP to see an effect.
Remember that there are settings in the script.