"Unspeakable Evil" is a common theme in topics dealing with villains or malevolent entities / people from theology and/or folklore - it refers to a phenomena found across multiple cultures in which the mere mention of the evil-doers' name is taboo and believed to bring bad luck to those who utter it or (in extreme causes) cause death, disease or other supernatural curses. "Unspeakable Evil" can also apply to individuals/beings who have gained such a level of infamy that the mention of their name, while by no means causing harm, is shunned upon - society as a whole (whether real or fiction) prefering to try and forget the evil-doer ever existed.
Simply ruined innocence of/corrupting many people is not enough for a villain to had Unspeakable Evil reputation. Many will achieve this by committing atrocities in apocalyptic proportions (destroying entire planet/star system/galaxy/reality for example) that resulting the hero/heroine made the truce with other villains against them. Not surprisingly, many of Masters of Horrors can achieve this, even more than typical Complete Monsters.
- Lord Voldemort (Harry Potter) - one of the most famous examples in modern media.
- Nuckelavee - An evil sea-spirit of Orkney whose name is considered unlucky.
- Candyman - A twist on the theme in which uttering his name three times results in the appearance of a malevolent spirit.
- Darth Nihilus - A Dark Lord of the Sith capable of destroying entire planets. This may have turned the whole galaxy into fear of him and most do not speak his name due to what he did and his great power and another one of the most famous and popular examples in modern media.
- Herobrine - An urban legend from Minecraft. Some people do not speak his name in a select hand full of cases in fear of him attacking them.
- Bloody Mary (folklore)- an old urban legend that inspired the Candyman and works in a similar manner, only the evil spirit in the legend is normally female.
- Satan - many people do not speak Satan's name, instead coming up with an alternate name (such as The Devil, Old Nick, etc.) (In The Powerpuff Girls, all the evidence implies that HIM may be The Devil but as he is so evil his true name was never revealed.)
- Hades - in ancient Greece Hades' name was rarely mentioned.
- Darth Vader - A Dark Lord of the Sith who is one of the most famous examples in modern media.
- Diablo - The titular antagonist's power to instill fear to his foes is said so immense to the point that his name was fearful to hear.
- The Knights Who Say Ni - They were feared due to anyone whom meet Knights Who Say Ni are rarely able to survived the encounter to tell the tale. Ironically, unlike other villains/antagonists whom listed here, Knights Who Say Ni themselves not so scary.
- Moo - The evil hybrid monster's name was rarely mentioned.
- Speakable Good - Heroism Wiki (For This Good Counterpart).