Source: Occult Bestiary, pg(s). 36
Upon becoming an overlord, a neothelid's head splits into two (each containing half of its intellect), its skin becomes darker, its tail splits into three parts which claw into the soil, and its body sprouts multiple squirming tentacles that seem to have minds of their own. The neothelid also becomes stronger and faster than its size would indicate. A neothelid overlord measures 100 feet long and 100,000 pounds on average, although the standard deviation is significant.
In some cases, when a powerful neothelid devours another of its kind, the predator's and prey's minds merge together into a single consciousness in a single entity. However, this evolution is not guaranteed, and in modern times, neothelid overlords are rarely created, as the neothelids are unwilling to practice cannibalism lest they go extinct.
Neothelid overlords become tyrants over their race, and keep lesser neothelids in line through seemingly random, punitive displays of power. Their authority allows a relatively large number of neothelids to co-exist in peace within a single settlement. For the most part, they are unconcerned with their lesser kin's wars against hostile species, like intellect devourers.
Neothelid overlords worship Shub-Niggurath, the mother of their race, and are actively plotting to bring her back to Golarion so she can unleash a new neothelid brood, resulting in an apocalypse greater than any since Earthfall. This plan is opposed by the elder things, who have fought neothelids in the past, and are manipulating patsies into opposing the neothelid overlords.