The story takes place long before the events shown in Of Orcs and Men. Styx, the first Goblin, wants to steal the heart of a World-Tree enclosed inside the huge and terrifying Tower of Akenash. It is a kind of Bildungsroman that leads Styx to a better understanding of his own origins. Source: Wikipedia, "https://en.wikipedia.org/wiki/Styx:_Master_of_Shadows," available under the CC-BY-SA License.