Using the zalgo generator
Type on the left. The marks pile up on the right, and Marks per letter runs from 1 to 15, though anything past about 8 starts climbing into the lines above and below until the paragraph closes up into a block.
Hang marks decides whether the growth goes up, down or both ways. Swap the boxes to strip them off. That works on zalgo made somewhere else too.
How the text overflows
Unicode has a category of characters that render on top of the character before them rather than beside it. They're called combining marks. They exist so a language can build an accented letter out of parts instead of demanding a separate code point for every combination anyone might need, and an e followed by U+0301 is an e with an acute accent.
Nothing in the standard caps how many you can stack. Hang 40 marks off one letter and a renderer will dutifully draw all 40, straight through whatever is above and below. That's the whole trick.
The characters underneath are untouched. Strip the marks and the original sentence is still there, in order, which is why the swap direction works at all. Screen readers vary, and some will read every mark aloud by name.
The most famous answer on Stack Overflow
Zalgo started in 2004 as a horror character in edited comic strips on the Something Advice forums, credited to Dave Kelly. The text effect took the name later and kept the tagline, "he comes".
Its best-known outing is a programming joke. In 2009 a user called bobince answered a Stack Overflow question about parsing HTML with regular expressions by refusing, at length, in character. The answer descends into zalgo mid-sentence as HTML and regex are declared to have summoned something. It's been upvoted more than 4,000 times and gets cited in earnest whenever somebody proposes regex for nested tags.
The joke has a real point underneath. HTML nests to arbitrary depth and regular expressions can't count, so the parser you want is a parser.
Related conversions
Strikethrough uses the same mechanism with restraint, hanging exactly one combining mark per character instead of 15. Upside-down text gets its effect from substitution rather than stacking, so it stays on one line.
To see the code points behind any of this, paste the result into the Unicode converter.
Questions
What is zalgo text?
Ordinary text with a stack of Unicode combining marks hung off each character. The marks render above and below the line, so the text appears to be falling apart.
How do I remove zalgo text?
Paste it above and swap the boxes. Every combining mark is dropped and the original characters come back in order.
Why does zalgo text break some websites?
A stacked character can be many times taller than the line it sits in. Layouts that assume a fixed line height overflow, and some input fields reject the length once each visible letter costs a dozen code points.
Can I use zalgo text in a username?
Often not. Most platforms normalise or strip combining marks in names precisely because of the layout problem, though bios and message bodies usually allow them.