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.
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.