The "NotThem" (as referred to by Adelard Dekker) is a shapechanging creature with the ability to alter memories of those who knew its victims. It then replaced them, living an apparently normal life, until moving on to a new victim. Some people are immune to its reality-distorting effects, and it takes a great deal of pleasure in tormenting them.
In its natural form, it is described as "long and thin", with "stick-like limbs" with "too many joints". Even when encountered in a monstrous form, it was described as looking similar to its victim but "stretched out".
In 1994, Rose Cooper is replaced by the Not-Them in the form of Not-Rose. Only her daughter, Lucy Cooper, is aware of the change (MAG 77).
In 2001, Adelard Dekker bound the entity (then in the form of Not-Carl) to a web patterned table (MAG 78).
In 2006, Amy Patel saw the NotThem become Not-Graham due to Graham's possession of the table (MAG 3).
In 2017, Jonathan Sims becomes suspicious of Not-Sasha after some unusual comments from Melanie King. He breaks the table trying to destroy the creature, but only succeeds in releasing it. He is pursued into the tunnels beneath the institute by it (MAG 77, MAG 78, MAG 79).