This is Fenrir, a monstrous wolf that is the son of Loki. Shackled by the gods, Fenrir is immensely strong and deeply feared by the gods. On Ragnarok (Doomsday) he is predicted to break free of his bonds and devour the earth and sky.
Any light will charge the pendent. However, direct sunlight, high wattage bulbs will help it reach its the maximum glow. The glow will gradually fade until recharged.