Black Skeletons with purple fire...
Hmm... Azurian Bonemen from the void, ruled by Lord Azurewrath.
The witch could possibly be a servant of Lord Azurewrath and summons the Bonemen to fight for her. Meanwhile in the void, all of the dead victims caused by the Bonemen are pulled down into the ground with her magic and are transformed into Bonemen, who then must suffer an eternity in the void until he/she can summoned by another Azurian servant.
The Void is a dark cloudy desert with solid black sand everywhere, with roaming Bonemen and old destroyed homes that used to be on earth, burned, and has each family that lives in each house as Bonemen. While in the center of this desert, visible from anywhere, is a massive castle engulfed in purple flames, where Lord Azurewrath sits on his throne.
There are paved stone roads running everywhere coming from each house like veins that all lead to the Azurian Castle, as if it were the heart. And maybe only one living soul, a human from earth, who has entered the Void Lord Azurewrath.
The sky is a giant spiral, like a hurricane, and the eye stays on the castle, where the dark light shines, while the life source is a gigantic ball the size of Pluto, entirely made of human (Or partially human) souls. The clouds that make this spiral are black and violet lighting strikes in random areas, only affecting any kind of life it can hit.
The bigger the lifeless buildings are, the closer it is to the castle. The castle still remains the widest and tallest building within the Void. This creates a super city teeming with Bonemen thirsty for living human blood. Some even bigger than others. Some even have armor and are more intelligent. And some aren't even humans, but animals such as dogs, horses, lions or even elephants. Usually domesticated by Bonemen.
I don't know, that was what this picture made me think of.