The Winter Court is one of the seven courts in Prythian. This court was one of the three courts along with The Day Court and The Summer Court to rebel against Amarantha's control over Prythian. The High Lord of the Winter Court is a pale-skinned man with a crown of ice.
Plot
A Court of Thorns and Roses
In A Court of Thorns and Roses, a contact from the winter court informed Lucien/Spring Court that The Blight took out two dozens of their younglings. Later in the book, The High Lord of the Winter Court watched Feyre's trials and bestowed a piece of magic to heal Feyre and turn her into a high lord.