Crimson Moon

Ultimate One. Of all the True Ancestors, Arcueid is said to be the closest to the Crimson Moon. Specific details are unknown, however when she transformed into the Crimson Moon [in Melty Blood] it would appear that her attack names were already prepared. Also when Zepia transforms into TATARI, the Crimson Moon can be seen overhead. This is quite odd seeing as there is supposed to be a contract he made with Altrouge which entailed Altrouge turning him into TATARI until the next Night of the Crimson Moon 1000 years later…

MB Official Strategy Guide: Melty Blood Glossary

Another name for the organism that was used as the template for the True Ancestors. Also known as the Crimson Moon Brunestud.
After he vanished, individuals close to him in power started being honored with the title of Brunestud.
However, in the long history of the True Ancestors, only two have ever received the name.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary