Slave Empress Tabikat is a boss in BLACKSOULS 2 you can find and fight inside the Queen’s Castle. She’s together with her husband, Sibimet, and they will attack you together in the throne room that’s right before the cell where Leaf is kept.
Tabikat is most likely based on a character with the same name from Lewis Carroll’s book Sylvie and Bruno.
They’re difficult to defeat if this is your first cycle. Killing her will give you the Soul of the Empress.