Tatari

Another name for the materialized Night of Wallachia. The origin of this name comes from the Japanese word for “curse.”

MB Official Strategy Guide: Melty Blood Glossary
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A system that incites fear and rumors among the people, circulates them, and finally cultivates them into a single, well-defined “rule.” It's like an urban legend that crops up one summer, explodes in popularity, then fades away as if it never happened at all.
The incarnation and realization of this system is known as the Dead Apostle Tatari. Because its form and abilities vary every time it appears depending on the scale of people's rumors, it is considered to be a perfect “unknown”—something with no true identity at all.
Whether it manifests as an enormous beast big enough to smash an entire mountain village with a single step, or a real-life bloodthirsty killer, the only sure thing is that the community that gives the Tatari form will be totally annihilated.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary