The Limbo is a semi-physical stage between life and death.
The Limbo is assumed to be a blank, white canvas, although no one has ever seen it. It has absolutely nothing but the lifeless souls that roam around it.
Unlike Hades, the Limbo cannot be access by a necromancer. Only a god is able to travel between the Limbo and Mythos and back again. Once within the Limbo, there is no possible escape, unless you are a god.
The Limbo has a large magical shield that holds back everything except beings of pure primordeal matter, like a god.