A less complicated approach would be to create a new mob in the mob db with half of its normal HP and on the death event you would respawn it and move it to the new location
Hercules currently doesn't trigger events when a mob is hit by an attack, only on-death events. Although I think there's a plugin that has OnNPCAttacked or something like that, but triggering an event on every hit would badly affect performance... A workaround could be to spawn a mob with the monster() command, retrieve its ID and then setting a npc timer that checks every second the UDT_HP of the monster and if (HP <= (MaxHP / 2)) you use unitwalk on it. The NPC should also check the death event in case an attack is powerful enough to kill it
