It's real Unicode, not an image or a font#
"Zalgo text" gets its name from a creepypasta character, and it works by legitimately abusing a real Unicode feature: combining diacritical marks, the same mechanism that stacks an accent onto a letter like "é". A normal accented letter uses one combining mark. This tool stacks many — a random handful placed above, below, and through each character — which is exactly what makes the result look like it's glitching or falling apart. It is still plain text underneath; a screen reader or a search engine sees the original letters with a pile of extra marks attached, not corrupted data.
Why intensity matters more here than in any other style on this site#
Every other text-style tool here maps one input character to one predictable output character. Zalgo does not — each mark is chosen randomly per character, and the mark count scales directly with the chosen intensity, from a couple of marks at "mild" up to over a dozen per character at "max". At max intensity, the accumulated combining marks physically overlap far enough that many apps and fonts struggle to lay them out cleanly, sometimes causing visible lag while scrolling past a large block of it. That is inherent to the effect, not a bug — genuinely excessive zalgo text is a known nuisance in chat apps for exactly this reason.