Some triggers for procedures i've thought of

Entity Hit:
(entity, targeted or source) is hit, do:
(entity, targeted or source) is hit with damage type (type), do:
(entity, targeted or source) is hit whilst (external trigger, effect or variable) is (true, false, a number, active or de-active), do:
(entity, targeted or source) is hit

Nullifying Damage:
Nullify (percentage) of incoming damage
Nullify (percentage) of outgoing damage

Damage Data:
Incoming Damage
Outgoing Damage