SCP-4666 is currently believed to be a single, exceptionally long-lived humanoid entity of unknown origin. Survivors of Weissnacht Events typically describe SCP-4666 as a very tall (between 2 m and 2.3 m) elderly male of European descent, with an extremely emaciated appearance. The entity always appears completely naked, even when observed outdoors in freezing weather.

Though the nature and extent of its anomalous properties remain uncertain, SCP-4666 appears capable of instantaneous or near-instantaneous travel to any location north of 40°N latitude, and possibly to any location on Earth.

SCP-4666 activity occurs exclusively within a period of 12 consecutive nights every year, from the night of December 21-22 to the night of January 1-2; this period is known as SCP-4666’s “active phase”. During this phase, in what are termed "Weissnacht Events", SCP-4666 will appear at dwellings in one or multiple locations north of 40°N latitude. In all known Weissnacht Events, these dwellings have shared the following characteristics: isolated rural location, home to a family with at least one child under the age of 8, and situated in an area with snow cover lasting throughout the duration of the event.