Magical Power

Magic is a mysterious energy that Humans, Elves and Beastkins can use in the new world.

The people who can use magic are the magic swordsmen because they use swords when they use Magic.

Dark Knights are swordsmen/swordswomen who imbued their swords with magic.

History
It is theorized by the Cult of Diablos that 10 millions years ago, the world of "The Eminence in Shadow" originally did not possessed magic. That is until the realms collided, with one of these realms pouring magic into the world. The Dragons went extinct due to their inability to adapt to the magic while the humans, elves and beastkin thrived.

Weaknesses
Constant abuse of magic would result in magical overload in which the users suffer horrendous mutations unless they find a way to control their magic.