> May - Má (nominative, not the correct form here)

Sorry; that's an error. A verb doesn't have a nominative
of course :-) I meant to say 'indicative'. The correct
form here is the subjunctive.

Another solution would be to do without 'mega' altogether
and put the main verb in subjunctive to convey the 'may...'
meaning. Futhermore the article might simply be dropped:

"Ár roðni."

alternatively:

"Roðni ár."

When it's put that way it sounds like it's straight out of
Busla's Prayer - a powerful curse from Bósa saga ok Herrauðs.

Here's a part of one strophe that illustrates this sentence
structure (noun in nominative + verb in subjunctive) very well:

Villist vættir,
verði ódæmi,
hristist hamrar,
heimr sturlist,
versni veðrátta...

May wights get lost,
May monstrous things come to pass,
May cliffs shake,
May the world grow mad,
May the weather get worse...

It goes on and on. It seems to me that "May the rivers run red with blood"
would fit in nicely :-)

Kveðja,
Haukur

P.S. Can't resist quoting some more of Buslubæn:

Tröll ok álfar
ok töfranornir,
búar, bergrisar
brenni þínar hallir,
hati þik hrímþursar,
hestar streði þik,
stráin stangi þik,
en stormar æri þik,
ok vei verði þér,
nema þú vilja minn gerir.

Trolls and elves and magical witches, farmers and mountain giants burn thy halls.
May rime-giants hate thee, may horses penetrate* thee, may straws sting you
and storms drive you mad and woe unto thee unless thou doest as I will.

* Can be substituted for a four letter word if one is so inclined.