Death Knights of Niflheim

The Death Knights of Niflheim were a group of powerful necromancers from the realm of Niflheim.

Power Rangers WarForce
Although they are not directly mentioned, a fraction of their power is contained within Kalin's morpher, which allows him to morph into the Frost Ranger.

Power Rangers Solar Knights
The Dark Elves of Svartlheim obtain their power and become Death Knights.