Natural Invulnerability

A form of Monster "Armor."

Description from the Dor Un Avathar

This type of protection differs from Invulnerable Armor in that each point of Invulnerability covers the entire creature, just like the Protect spell. Unless detailed otherwise in the Monster entry, Invulnerability cannot normally be Healed or Mend in any way (the main exception to this are Amorphous type creatures, q.v.)

Description from the Celestial Codex

Functions exactly like Invulnerability (above) except it may be Healed like Natural armor at the rate of one point overall per Heal ability or spell. Most creatures with this type of armor are also Amorphous (see Traits, next page).

Not to be confused with Natural Invulnerable Armor.