[quote=@LadyRunic]
[@FateWeaver] Could you tell me what that is exactly, please? I can't find a reliable source on it.
[/quote]

Essentially, it's the Cheshire Cat of the Fae realm. Typically a direct servant of the Unseelie queen/king and particularly malicious individual. It's true form resembles a large black cat with bright gold eyes and features akin to small predatory cats like lynxes. The Grimalkin is also called the King of Cats.

The fae cats in general are better known as Cait Sith or Cat Sidhe, but the Grimalkin is generally an older and better-known member of this race. They are generally associated with the Winter Court and are according to legend attracted to warmth. Fabled to steal the souls of the dead, Cait Sith are supposed to be 'distracted' by riddles, catnip, and music.