What is the ruling on being named "King of Darkness" or "Prince of Darkness," and is there any objection to that, especially since darkness belongs to Allah Almighty? And what must the questioner do if this is forbidden?

It is forbidden to be named "King of Darkness" or "Prince of Darkness" because it is a title famously associated with a fictional cartoon character that contains superstitions and polytheistic elements. Adopting such a name promotes falsehood or suggests approval of it. Entering forums with pseudonyms is permissible, provided that the name does not contain any Shariah-forbidden elements.