Darkness is vulom. Master is in as in "master archer", or thur as in 'slavemaster'. I will use in below.

"Darkness's master" is most literally vulomro in. This phrasing is rare, but use it if you need to preserve the word order. As in English, the genitive ending can be omitted to give a direct apposition (vulom in), which can also become a compound (vulomin).

Taking the other word order, in do vulom is the most neutral equivalent to "master of darkness". A -se- compound is also possible (insevulom). You could separate it to in se vulom, though this is atypical usage. Note that these forms can be preceded by the honorific article faal as they allow it to unambiguously signify the person.

So, from most to least typical:

in do vulom
insevulom
vulom in
vulomin
in se vulom
vulomro in