Laravel's answer I think is best for your precise question, but I note that there's a bit of ambiguity with the people here. You have on the one hand the Night Lords and on the other hand the Lord of the Night, but in Latin their titles would be rendered without distinction.
If you wanted a coherent system in place, the "scary, stealthy dudes" should be domini noctis ('lords of the night'), which is the most straightforward way of rendering "night lords." The Romans would have read that as people who have some mastery over the night (cf. dominos insularum in Suetonius' Caesar 41 or dominum rerum suarum in Cicero's Tusculan Disputation 3.5.11). I suppose theoretically they could be the night's lords (a subjective genitive), but it doesn't feel natural to me to read it that way.
If they are merely associated with the night, especially if their activities happen at night, domini nocturni is an attractive alternative. You have e.g. the fur nocturnus ('nocturnal thief') mentioned by Phaedrus and elsewhere.
Yet you also have, found in Plautus, the Nocturnus, "the nocturnal one, i.e. god of the night." He's not really well attested, but fun the passage he seems to be the one in charge of the night.
So if you reserve the title "lord" for both their god and themselves, the distinction is muddied. Moreover, dominus as a title or epithet for the gods isn't common before Christianity. You have a few solutions that make their relationships and titles clearer and more classical.
One solution is to keep dominus only with mortals (the domini nocturni or domini noctis), and adopt the title Rex for their god. In fact, in Lucan we already have Rex Noctis as a title for Hades (6.741). If you wanted to go with Nocturnal instead of Hades, you could still do Rex Nocturnus.
Alternatively or additionally, you could also change the Night Lords into something else. If they have a religiosity to them, the Flamines Nocturni sounds super cool. To emphasize their fighting prowess, perhaps consider Bellatores or Milites. If they're in service of him, then Legati is an option, or perhaps to emphasize their mastery, Magistri could be chosen instead. The latter two have non-military uses, though, so they sound less cool.
One group that might interest you are the Tresviri Nocturni:
The officers here mentioned were called "nocturni Tresviri." It was their province to take up all suspicious characters found abroad during the night. They were attended, probably, by lictors, or subordinate officers, who are here referred to as "homines octo validi," "eight sturdy fellows."
For the imperatives of the above suggestions, the only words you'll need to worry about are the following two, and only in the singular:
- Dominus -> Domine
- Nocturnus -> Nocturne
This is for direct address. Examples (Ave or O +):
- Rex Nocturne (King Nocturn)
- Domine Nocturne (Lord Nocturn)
- Domine Noctis (Lord of the Night)
- Domini Nocturni (Nocturnal Lords)
- Domini Noctis (Lords of the Night)
- Rex Noctis (King of the Night)
As an unrelated side note, Gothic is not a made up language, but a real one that unfortunately was not well preserved. In language classification, the term "high" refers to geographical location: Old High German was spoken in the south of Germany leading into the Alps; Low German was spoken among the flatlands of northern German.1