The Terrible Trivium is a demon in the Mountains of Ignorance that has no face and tricks unworthy people who enter the mountains. He is a notorious time-waster who fooled Milo, Tock, and the Humbug into doing things that would take eight hundred and thirty-seven years to finish. They finally escaped by fooling the Trivium into counting pebbles. He wears a dark suit with a well-pressed shirt and tie, polished shoes, a white handkerchief in his pocket, and a well-brushed hat. He likes doing things that are not important. His nails are clean. He is well mannered but wants everyone to do trivial tasks and not important ones. He made Milo move some sand with tweezers, Tock fills a well with a dropper and the Humbug make a hole in a cliff with a needle. He is the demon of petty tasks and triviums. His end comes when the word "industry" falls on him.
- In the movie, he looks similar to his book counterpart, save for a missing neck, wrists and ankles.