Who are "Those Who Must Be Kept" in the novel, The Vampire Lestat?

Those Who Must Be Kept are Akasha and Enkil, the queen and king of all the vampires. They are almost six thousand years old, alive during a time even before the pyramids were constructed. After many battles over so many years, they ceased to move or speak. Due to their lack of mobility, they need to be cared for, and Marius inherits this responsibility.