The "Moral Event Horizon", or "MEH" for short, refers to an action by a villain that is so evil that it usually eliminates audience sympathy for the villain and makes it clear that they are unrepentantly evil and will not be redeemed, unless if they themselves are offended by the actions of another villain, or admit themselves that they are beyond redemption. The scientific fact that nothing can escape a black hole is referred to as the boundary of the region from which no escape is possible and is known as the event horizon, hence the term Moral Event Horizon. A villain who crosses the horizon, or enters the black hole, has no hope of escaping except under extremely rare, extraordinary circumstances. All Complete Monsters are by definition beyond the Moral Event Horizon. However, crossing the Moral Event Horizon does not necessarily make a villain a true Complete Monster, it only means that they are irredeemably evil. Some villains have or almost crossed the Moral Event Horizon, or done something despicable enough to make the fans believe they did so, only to later redeem completely or partially.
A strong example of a Complete Monster would be either a villain who (in addition to having no or a very poor excuse for being evil) commits multiple especially heinous Moral Event Horizon-worthy deeds, or if the line in question proves to be heinous enough. The notable consequence on the villains' reputation whom crosses MEH would led either mere mention of their name as a taboo and believed to bring bad luck to those who utter it (or in extreme causes disease, death, or other supernatural curses) or they themselves became Damned Souls when they died. By definition, a character can only cross the Moral Event Horizon once; however, it is possible for a villain to have more than one event as a potential Moral Event Horizon, such as the various adaptations of the Joker and Teridax. Other villains have more than a potential and often leave an impact on the public that they do wicked things all the time and commit Moral Event Horizon acts like a hobby, such as Lord Voldemort, Sir Isaac Ray Peram Westcott, Emperor Palpatine and Emperor Tathagata Killer.
These characters are almost invariably Complete Monsters.