The Lady of the Cold, also known as The Great Cold (Swedish: Isfrun or Den stora kölden) is a supernatural being appearing in Moominvalley in Moominland Midwinter. The Lady of the Cold is in many ways cold and winter anthropomorphized, having a mysterious and almost godlike presence. Every winter, she passes through Moominvalley, bringing icy cold temperatures with her. Her appearance is described as very beautiful, but if she looks someone in the eye, that person will instantly freeze to death. Making her similiar to mythological creatures such as sirens. It is as such very important to stay indoors and keep warm the night she arrives. Her cold properties resembles those of the Groke, though instead of freightening people by her looks, the Lady of the Cold lures people towards her with her appearance. In the 1990 anime it is revealed that even the Groke fears the Lady of the Cold.
Unlike most key characters in the Moomin books, the Lady of the Cold is never shown in its illustrations. This was likely a consious decision by Tove Jansson in order to make the character more enigmatic. The result of this has been that every adaption of Moominland Midwinter has presented widely different incarnations of the character, designed entirely by the producers of that particular adaption.
In the events of Moominland Midwinter, Too-ticky advices every little creature to stay indoors when the Great Cold arrives, including the absent minded Squirrel with the Marvelous Tail, who is more interested in finding a lost pine cone than listening to Too-ticky's advice. When night comes and the Lady of the Cold arrives, the squirrel has forgotten to stay indoors and gets confronted by her, instantly killing him. This makes the Lady of the Cold, together with Edward the Booble, one of the only two characters in the Moomin books who has succesfully killed someone. A funeral for the squirrel is held by Moomintroll, Too-ticky and Little My. Unable to burry him, they get unexpected help from the snow horse built by Too-ticky for the Lady of the Cold, who has mysteriously come to life. The horse walks the company to the beach, where it takes the squirrel and gallops towards the horizon.
In the 1990 Japanese anime series, the squirrel is absent. Instead Little My attempts to get a close look of the Lady of the Cold's face without being frozen, she succeeds at this by windsailing past her at high speed. This angers the Lady of the Cold, who gives life to Too-ticky's snow horse, chases Little My down on horseback and freezes her solid. Moomintroll and Too-ticky instantly proceeds to heat Little My up again, who miraculously survives without a scratch.
- The Icelandic band Kælan Mikla took their name from this character, using the Icelandic translation for Lady of the Cold.