You've actually done that rather well. It is a very nasty piece of text to translate and any attempt to give it fluid meaning in English is probably pointless. It is, by design, cryptic. Here is my version, but I wouldn't say it is better than yours.
Even if the moon should burst with radiant plumes,
The dawn is still certain to come in the morning,
But over an infected land vile gods arise,
The sword of light, its wrath hitherto buried, hunts them,
The sword of darkness, that has become boundless, suppresses them,
Only one blade may determine the world's fate.
~From Louisoix's Memoirs
Line 1:
Gleißen = to gleam, to glisten, with flames. A verb, it is referring to the way in which dalamud "bursts" open, suggesting beams and rays of bright light. Thus my choice for "with radiant plumes" to convey that sense of rays of fire.
Line 3:
Ödem = Edema = An unusual retention of fluid in the body tissue. Hence my translation of "infected".
Wüst = Harsh, like the nastiness of the desert. It can mean desolate, but also wild or possibly forsaken. Sounds like the primals, so I used "vile" and made sure "gods" was not capitalised.
Line 5:
Tilgt = Blotted, kind of like an ink blot. In the context, I've gone for "suppress".
Line 6:
Los = Lot, as in drawing lots or your lot in life. Hence the contextual choice to use "fate".