The Mirror is an object of unfathomable power, being able to observe "Everything" anywhere, anytime, anyplace. Only the most powerful gods of pure heart are able to even step within the room this object is held in.
Universal Observation Edit
The Mirror has the ability to see all things that exist and will ever exist.
Presence Restriction Edit
The Mirror prevents anyone not of worthiness from even entering the presence of it.
When Gaster tries to locate Chara using the Mirror, He senses a foul presence within it. It is later revealed that Chara has corrupted the Mirror, and later uses it to trap Gaster and scatter him across space and time.