During the Red Raids, the Utaru were one of many tribes that fell victim to the Carja's brutal raids and sacrifices. The Utaru tried to appease the raiders with gifts of grain, but suffered slaughter and slavery by the Carja under the 13th Sun-King Jiran.
In The Old Ones (Datapoint) it is written that the Utaru believe those who die are returned to the soil-- nourishing the land, and living on through new growth. As long as the dead are remembered, they will contribute to the harvests and make them bountiful.
The Utaru do not feature heavily in Horizon Zero Dawn. They are referred to as a distant tribe.