Kur is the Mesopotamian underworld, also known as Irkalla. This is the realm that is ruled by Nergal and Ereshkigal, as well as a “shadow cabinet” of lesser deities.
Kur is not a happy place. It is home to many demons, some of which tasked to keep souls of the deceased trapped in there. These are named the galla.
There are many topic in Mesopotamian mythology that can be considered overly complex. Kur is not one of them. Unlike in other mythologies, all souls are destined to end up in Kur. It does not matter if you have lived a good life or a bad one, the destination is always the same.