The story
Said to have been coined in 2004 by Dave “Shmorky” Kelly as the name of a mysterious malevolent entity appearing in webcomics and creepypasta.
Etymology from Wiktionary, CC BY-SA 4.0.
zalgo
Today it means: In full Zalgo text: digital text to which random Unicode diacritics have been copiously applied, obscuring the text, often protruding out of expected text boundaries, and imparting a "cursed" feeling. Full definition →