The Unyielding Terror
From Destinypedia, the Destiny wiki
The Unyielding Terror is completely immune to damage of all types and must be avoided, however it cannot leave the section it was spawned in. The moment the Unyielding Terror spots a player, it immediately rushes towards them and tries to kill them. Once it destroys all threats, it either wanders its section of the branch, or if a dead Guardian is in the area it resides in, it waits by them in case any other Guardians come and attempt to revive, but if another enemy is spotted, it will gladly abandon its post over a Guardian to chase the new foe.
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