Zalgo text
Text with many Unicode combining marks stacked on it to look corrupted or cursed, named after a mid-2000s internet horror meme.
Zalgo text is text that has had Unicode combining marks stacked onto its characters — often dozens per letter — until it reads as broken, corrupted, or "possessed." The name comes from a mid-2000s creepypasta/internet-horror character called Zalgo, associated with the idea of reality slowly corrupting.
Mechanically it's identical to what most people now call "glitch text": each visible character keeps its extra combining marks stacked above, through, and below it. See Unicode combining characters for how that stacking works.
Generate your own with the zalgo text generator, which defaults to a heavier preset than the plain glitch tool since "zalgo" searches usually want the classic extreme look.