Oculus of Abaddon

This artifact doubles the duration of all spells cast by its wearer as though using the Extend Spell feat. Three times per day its wearer can peer into it to cast true seeing spell as a 20th level wizard. Yet, staring into this gem is dangerous: whenever the wearer uses this ability, they must succeed on a Wisdom check (DC 15), to avoid seeing something truly unsettling... Or worse – be seen by it.

A corrupted gift the Four Horsemen gave their loyal servant in reward for his myriad atrocities in ancient times. The full power of the Oculus is not known, but it has power enough to destroy entire cities. It has power over the mind, and it is capable of revealing what is hidden, though not everyone can use it to its full extent. Revealing the true power of the Oculus requires a sacrifice, inserting it into the owner's eye socket. The suffering associated with this merger can be so strong that it leads to madness. The Oculus can also accumulate power and release it in horrible acts of magic, but that is a lengthy process that can take centuries.