The Strange Orb is a light pet summoning item that summons an ocean spirit to follow the player. It is obtained from the Shadow Chests in the Abyss. When equipped while the player is not submerged in a liquid, it sleeps while holding the orb and gives a small amount of light. When in liquid, it swims above the player and provides much more light.
The Ocean Spirit is a very effective way to increase your light level in the Abyss.