Dark ore can be found in the wreckage of the alien starcrafts when you defeat the alien, and you can exchange 10 Dark ore and one Meteor for one Core of the same category.
The attribute and the weight of the Core come in random. For Core I - V, the effect ranges from 1% to 20% and the weight ranges from 5 to 50.
The nice core has advantages in both the effect and the weight, the lighter the better.
One light and nice Core can be created when one heavy Core with better attributes fuses with one light Core, and core fusion makes sense when the core with better attributes is heavier than the other.
The Core enhances the attributes of the hero by percent!