In this world, creatures called gate spawns would appear from thin air in the form of ripples called 'gates' and destroy everything in their sight. No one knew why and where they came from.
Some of them looked like earth creatures whiles some are different entities entirely. Their sizes also varied. But what was similar between all of them was how dangerous there were. Their strength was without a doubt far greater than any normal creatures on earth and whenever they appeared, heavy losses were recorded. Normal weapons couldn't affect them and just when all hope was lost, a dead gold tier gate spawn emerged from a gate. From the gate spawn , researches were made and a solution was found to the problem.
The skeletons obtained from this gate spawns could be melted and treated as metal to forge weapons which could harm other gate spawns provided the tier is lower or the same. This new armory was called dark metal. Not all dark metal is the same. They vary based on the tier of the gate spawn it was obtained from. Gate spawns were in three tiers. Black tier, silver tier and finally gold tier. Each tier was decided based on the colour of the gate spawn. The darker the colour the stronger the gate spawn.
Hence dark metal from a dark gold tier gate spawn was stronger than that of a light gold tier.
After dark metal's discovery, it became one of the most expensive items on the market. This was because it took a lot of resources and money to produce dark metal. Though some gate spawns were large, it doesn't mean more dark metal would be obtained directly. The process of making dark metal requires expertise, extreme temperature and pressure and other important factors.
As the danger in the world from gate spawn became unbearable, dark metal became a necessity and people did everything they could to obtain some of it.
In rare occasions, one might obtain the spawn essence of the gate spawn. The spawn essence is basically a small orb found in the corpse of the gate spawns. One could fuse with the spawn essence to gain a special attribute of the deceased gate spawn. This depends on the specialty of the creature. Hence it could be fire, ice, speed, strength etc. Usually there's only a single attribute obtained from the spawn essence. Obviously the stronger the gate spawn, the stronger the spawn essence obtained.
Obviously spawn essences were even more expensive that dark metal due to how rare they were and the attributes they could grant an individual.
In the course of the sudden appearance of gate spawns, special institutions were created all around the world to train special soldiers to battle against the threat of gate spawns. The special soldiers were known as spawn hunters.