Darth Rivan, an evil Sith Lord of that era, made him love the dark side of the Force more than the light. Then, when Michael was a chief of a large team of Jedi, he rebelled against the Jedi High Council and fell from his pride and hate with a very large number of Jedi who followed him to became evil like Michael. From then on he was named as Darth Horrendus, which meant "the horrible one."