[@CirusArvennicus]

Well, most guild names are typically "[Adjective] [Noun]" (Or just "[Noun]" sometimes, such as Saber-tooth, which is a hyphenated compound word. I don't know how they write it in the anime.)

Such as Lamia Scale (Scale is a noun, Lamia is an adjective describing what it is a scale of)

This is how I named my dark guild, Iron Enigma (Enigma is a noun(ish), and It's made of Iron.)

Phoenix Wing ([Wing] Noun [of a Phoenix] Adjective)

I figure dark guilds would have a more sinister tone in their name though.

Like Grimoire Heart, Grimoire is an evil-sounding word because it's associated with witches and "black magic".

So your best bet is that format if you want easy names that sound good.

Examples:
[list]
[*]Golden Crown
[*]Dragon Claw
[*]Black Amethyst
[*]Sky's Emissary
[*]Solid Tooth
[/list]

While you don't have to limit yourself to this format, it's pretty simple and you can make some cool names (Feel free to use names from my example list, I just made them all up on the spot). The ones in the explanation are taken.