Morlocks are a species of humans in H. G. Wells' first novel
The Time Machine. The novel is set mostly in 802, 701 A. D., when human beings have evolved into two species -- the
Eloi and the
Morlocks.
Morlocks live in a network of underground tunnels. They are generally a nocturnal species. Because they are rarely, if ever, exposed to sunlight, their complexion is almost completely devoid of color. For the same reason, they also fear sunlight and fire.
They tend the Eloi like cattle, feeding them and taking care of their basic needs. However, an unsuspecting Eloi could be stolen away on any given night to become dinner for the Morlocks. The Eloi are innocent, child-like people, who vaguely fear the dangers that lurk underground and in dark places.
The protagonist of the novel, known simply as The Time Traveller, uses his Time Machine to travel from the present day to the world of the Eloi and the Morlocks. He loses his beloved Eloi Weena to the Morlocks one night and sets out on a mission to save her from their clutches. It is in the course of this mission that he learns about the Morlock world, spoken of only in vague whispers, where the sun shines.
Addendum: The entire novel is not set in 802, 701 A. D. The Time Traveller makes stops in eras far ahead of that of the Eloi and Morlocks, when the planet is in utter decay with no visible signs of life. He also stops by in the 24th century, when returning to his own time. In the latter era, he gets waylaid by a fame-hungry man and nearly doesn't make it back.
I don't mention the other time periods in the original post because he meets the Morlocks in just one era.