Accumulates experience as you do. When fully charged it produces 400 Eyes of Q’bthulhu. This effect can only trigger once a week.
The Oculus Opus is a Tome bought in the Shadowy Market for 1 Lunar Soul. They produce 400 Eye of Q’bthulhu, when they are fully charged. Unlike Common Tomes (that can be used as often as you like without cooldowns), these tomes can only be used once a week, although, they provide more rewards than Common Tomes per individual use.