I should have few good Viking curses sitting around... I've got two tomes of Icelandic saga, after all. (I've read hardly any of them, but what I have is good readin'.)
Let's see... use verse. Long verse. Vikings were very fond of poetry, and the more of it the better. Here's one uttered by Egil Skallagrimson about the king of Norway, who had just ruled against him in a court case:
'May the gods get rid
Of this ruling robber,
Let the heavens hang him
For highway robbery!
May Odin and the others,
Frey and Njord, show their anger
To this enemy of ease
And order at Assemblies.'
Egil then sailed back to his home in Iceland just in case.
Egil was often introduced as a troll, by the way; he killed hundreds of men in his career and had a bone disease that made him virtually impenetrable to weapons. Big, huge, and about as manly as the vikings ever got, although not handsome by their standards. He'd utter verse at many occasions, and wrote an entire presentation in one night - it goes on for pages and pages - to save his life at one point.
Norwegian law forbade certain curse-related practices called shame-poles. (The actual word is niDstOng - the D is a soft 'th' D, and the O has an umlaut, for those interested in pronunciation. NiD covers infamy, slander, shame, derision and insult.) At one point, our man Egil set up a niDstOng on an island near Norway: he took a hazel pole, went onto a cliff facing the mainland, and jammed a horse's head on top of it. The next bit was described as an invocation:
'Here I set up a shame-pole, and I turn this shame against King Eirik and Queen Gunnhild.' Then he turned the horse-head to face the mainland. 'And I turn this shame against the guardian spirits who inhabit this land, so that they shall go astray, unable to detect nor discover their swellings until they have driven King Eirik and Queen Gunnhild from this land.'
He jammed the pole into a clef in the rock and left it standing there with the horse-head facint towards the mainland. Then he carved the whole invocation on the pole in runes. ...
I read that there's also a form of shame-poling called treniD (tree-shame) which involves carving human figures in compromising sexual positions. Also forbidden under Norse law.
So there's some authentic Viking cursing for you. It's not heat-of-battle stuff unless you're really good at on-the-spot versing (which should probably rhyme; it was originally spoken in some ancient language which I'm mangling quite horribly without the two dozen extra letters you find on a Danish keyboard). But it's what they used, and it could really add some flavour to fits of rage if, instead of smashing up a bar, you go out and kill some guy's horse and do horrible things with the remains.
Failing that, there's the threat of straw-death which the Vikings feared. If you died in bed you'd go to Niflheim, Hel's realm, and sit around in the cold forever. If you died in battle, Valhalla. There's bound to be a good curse in there somewhere... "May you die coughing and alone, battle-fearing coward!"
