Tales of Tolgard
Illithids goals include bringing the entire world under their dominion and eventually rejoining with the elder brain to form a communal consciousness. They certainly do not care about the number of sentient creatures they must kill to achieve their goals. This disregard for life extends to their own children as well. Sometimes their young must be sacrificed for a higher goal.
There are a number of creature that are resistant to illithids mental powers of persuasion. They have also created through experimentation additional races that prove resistant to their powers. Sometimes their powers are ineffective and other times they are simply too slow. In these cases nerve swimmers are needed.
Nerve swimmers are immature illithid tadpoles taken from the elder brain pool. These sentient beings can literally travel along a beings nerves to provide a level of exquisite torture beyond the abilities of the finest interrogators. As a last resort, they can travel into the subjects brain and eat any parts of the mind that still fight against the interrogator. These leaves a compliant prisoner but does irreparable damage to the them. More often than not the young tadpoles die in these efforts but there are always more to draw from.