4SRD:unbelievable toughness

4SRD > Unbelievable Toughness

Unbelievable Toughness: You have the unbelievable toughness Half-dwarven racial power.
Once per encounter, you turn invisible in response to harm.
(Immediate Reaction, Personal)
When you take damage, you become invisible until you attack or until the end of your next turn. This is an illusion.