The Sleeper itself is only encountered as the second boss in the 'Endless Harvest' mission. The Fracture, the first phase of the fight against the Sleeper.

Unfortunately, by unsealing the rune stones around the landscape, it allowed a horrific monstrosity to escape into the wilderness.

The heir sent their mercenaries into the area to investigate, where they were ruthlessly attacked by endless hordes of creatures under the Sleeper's thrall. The miller, his wife Mildred, his farmhands and even all of the animals and scarecrows became corrupted into creatures neither dead nor alive. Locations and creatures appeared and disappeared at a moments notice, while everyone in the area succumbed to the Sleeper's influence. The impact of the comet into the miller's home shattered the mill apart and the energies radiating from it transformed the landscape into unholy grounds where time and space became ever-shifting. Though he regrettably did not live to see his work bear fruit, the Ancestor's stones eventually succeeded in calling a being from the stars onto the Earth when his heir took control of the estate after his passing. Setting up a large circle of rocks inscribed with arcane runes around the farm's perimeter, the Ancestor did not actually intend on helping him however and merely used the miller's desperation for another cruel experiment. After a particularly terrible harvest, the miller of the local farm came to the Ancestor with a plea for aid, who happily obliged his request. Nothing is known about the Sleeper before it came to Earth, only that it spent some time drifting among the stars as a large comet. The tentacles it extends out of its shell in one of its attacks have purple skin. One eye is visible in the middle of its body, resembling the eye of a cephalopod. The Sleeper is an otherworldly entity encased in a cocoon made of rock and glowing green crystals.

Darkest dungeon ancestor wiki