The Invulnerability Hunter is a boss found in Doom 3: Resurrection of Evil. It is the final Hunter the player will encounter. Like the Berserk Hunter, the Invulnerability Hunter is noticeably larger than a Hell Knight, and has a bulging, extremely muscular body with a blue aura surrounding it.
The Invulnerability Hunter draws power from three active power coils in the room, rendering it invincible. Destroying the open coil will cause a temporary shutdown, leaving the Hunter vulnerable. When the remaining coils reactivate, repeat the process. When all the coils are destroyed, the Hunter will no longer be able to retain its invulnerability. One can hide from the Hunter by running underground, but it will stomp the ground to damage the player should he be standing up.
When it is vulnerable to attack use the Artifact's Helltime ability and pull out the chaingun and start firing until he becomes invulnerable. Then, in its invulnerability state, it will attack the player by shooting energy balls and electric shockwaves.
Killing it will gain the invulnerability power for the Artifact.
The power granted by is is similar to the Invulnerability powerup, just as the Berserk powerup, though intrestingly unlike the Berserk powerup, the invulnerability powerup makes no appearance in Doom 3.
Doom 3: Boss FAQ - posted by D00MBuggy.