History
The Third Eye of Horus was an item given to Beast by Doctor Strange during an emergency to allow the X-Man to see the Shadow Creatures of the Twelfth Dimension.
The first use of the Third Eye caused Beast to pass out. When he used it a second time, words spoken by Strange earlier allowed him to grasp a better control of the item and see the magical creatures in an appearance more suited to his scientific mind. After exiling the Shadow Creatures, Strange let Beast keep the mask.[1]Properties
The Third Eye of Horus allows individuals to see beyond the normal human spectrum. Common sides effects include abject eye-bulging terror, and loss of sanity. An inexperienced user is likely to pass out.[1]