Interesting question. Some similar, one-syllable: rel 'domination' or nax 'cruelty'. Other possibilities could also include lok 'sky', viin 'to shine', or kun 'light'.

Hmm... Those would shorten the phrase, but those definitions would throw it off by a longshot. 

To Further Explain what I see in the words as they are now, despite the consistency flaw, I must directly quote part of the connotation of Hevno. "Brutal or harsh, ... Describes severity in conditions, particularly weather..."

This, combined with the other two words being simply "Daylight" and "Beauty", gives me the visual of the beautiful daylight turning incredibly harsh, intense, and unforgiving; which is exactly what the Sunhallowed arrows do to the sun.

To contrast, my shout for the Bloodcursed arrows, Blood Darkness Sun specifically describes the sun itself turning bloody and dark, with the added bonus of a sense of doom. (I could have used Vol instead of Vul to play more to the horrific terror in seeing the sun turn red, but then I wouldve lost the aspect of a literal lack of light.)

However, to return to the main point, my problem remains finding a synonym to Hevno in the sense of harsh, severe weather, while only using one sylable. 

I do not know a good answer to your dilemma, but perhaps if a context is given to word paarth suggested, it could replicate, though not perfectly, the effect of hevno. Rel-domination, could imply that the daylight is stong, harsh, powerful, qualities usually associated with domination, though it lack the direct wether connotation of hevno. Nax-cruelty, could imply that the sun become cruel and harsh as well, perhaps like a desert (I have heard many people say the cruel desert sun), however other connotations of cruelty would probably be better suited to blood curse, and like rel, has no direct weather connotation. the other possibilty that paarth suggested that could make sense in the context of what you want is viin-to shine. It could imply that the sun is shining brighter, stronger, and seems to have a more direct weather connotation to the sun, as most people generally associate shine with celestial bodies like the sun, stars, moon, before other light sources-but it does not directly imply strength, harshness, or severity as you want. So if none of these words work when presented with these contexts, my recommendation would be just go for it and use hevno anyway. I do not think many people will be too offended by hevno having an extra syllable,  given that the extra syllale does not increase cooldown time.

Hmm.. Well, just as an afterthought, if I did end up going with Hevno, would it be considered cheating to simply shorten  it to "Hev"?

I'm not sure what proccess goes into the creation and acceptance of new words, but I could see Hev potentially becoming a sort of slang for Hevno that would retain its meaning.