The 99th dimension is something many people in Aetheria mention when referring to a death. Although not much is known about The Land of the Dead, it is known that it is the dimension where beings go after they die. There are no other known ways to get to this dimension.
All beings have an extremely small duration of time before they go to The Land of the Dead. This is the reason why overkill exists. The being is taking damage while dead before going to the 99th dimension. The 99th dimension can be assumed to be the last existing dimension in Aetheria.