Takala: The Littlest Giant
Nightmen legend that giants roamed the north, one was born too small and was named Takala (popular Nightman name).
Takala fell in love with the High Maiden, someone revered religiously by them. She came from the north, a human woman. Her father would not allow her to wed such a monstrosity, and this made Takala very mad. He worked up some of his larger giant brethren to overwhelm and destroy the village, and in the process, whisked the High Maiden away. This made the realm of men angry, forcing the giants ever south as wars escalated. Takala and his High Maiden fled to the mountains in the north. They gained followers and had many offspring. The people of this cavern are an offshoot of those northern giants and humans.