Lairs are optional, multi-stage combat encounters found in each region. They culminate in a unique Lair boss fight (e.g., Dreaming General, Harvest Child, Librarian, General's Bodyguard). Successfully clearing a Lair provides a significant amount of Baubles, Relics, and a guaranteed rare trinket. However, Lairs are incredibly challenging and will consume a lot of your party's health and stress. Only attempt a Lair if your party is in good condition and well-equipped. Consider the specific resistances and vulnerabilities of the Lair boss when choosing your party composition.
