Comment by db48x

Specifically _Vietnamese_ combining characters. The Vietnamese writing system uses multiple combining characters at a time, and stacks them vertically. Throw in a few that wrap around the character like t҉his, some 𝑎lternatᵉ lꬲttꬲr fᵒrms, disturbing imagery, and perhaps a few other tricks, and you have zalgo. See also https://stackoverflow.com/a/1732454/823846