Monoculus is one of the non-playable characters in Lego Dimensions 2: The Rise of Enoch, from the Team Fortress 2 franchise.
Monoculus was created when the young RED Demoman was sweeping then he found that Book that Merasmus told him NOT to touch, he touched it and his left eye was magically ripped out by the Bombinomicon, an evil grimoire detailing explosives. It had tempted the Demoman to touch it by luring it through a magical voice. The Bombinomicon's owner Merasmus the Magician managed to separate the Demoman and the book before it released his eye, keeping the eye in his staff to prevent it from escaping. Many years later Merasmus ended up roommates with the RED Soldier, whom he did not share a very friendly relationship with. After an instance of arguing during the RED team's Halloween party, the Soldier breaks his staff in half. In retaliation, Merasmus summons the now released Monoculus against the mercenaries.