Where Zalgo comes from
“Zalgo” began around 2004 as a creepypasta meme in which ordinary comic strips were edited to become increasingly corrupted. The name stuck to the text effect, which uses Unicode combining diacritical marks (U+0300–U+036F) stacked far beyond what any language needs.
Be kind with it
Heavy Zalgo can spill over lines of text above and below it and is unreadable to screen readers. It is fun for a username flourish or a spooky message; avoid it for anything people need to actually read.