Almirian shadow guards.
Arkus.
Trojans are the "Dark Knights" you speak of. That's even their filename.
They should have different types of gear but they all have an eerie purple personal colour and voodoo-cursed eyes (I know noting like that exists but it would be awesome) and also a cursed runic aura of sorts. Instead of element types there should be different sorts of knights, like strikers, gunslingers, bombers, defenders, etc.
This kinda sounds familiar... ya' know, dem annoying Phantomz in da graveyardz,
So undead themed version of mecha knights? Needs more details. What kind of attacks does it have? Does it do pure shadow damagr? Do split normal and shadow adamage? Or both? There are a few things I missed but those are the several main things that I want to know.