The Altar Lamp is a light-emitting lamp block from Vethea.
The Altar Lamp emits light in Boss Chambers, such as in the Raglok Chamber or the Wreck Hall, up on the highest layer (Layer 4) of Vethea. A few of them spawn near the Altars. However, since the floor of the adjacent battlefield is made partly up of light-emitting Firecrystals, the Altar Lamp's lighting purpose is mostly nullified and it becomes a decorative block.
Altar Lamps can be broken and harvested by hand. They can then be moved to your Vethea house.
The block itself is actually called "Altar Lamps."