At level 720 the Valhalla Vikings will begin to appear on your Battle List, and are great for fighting at higher levels when living opponents begin to seem fewer and far between. You'll be able to tell the apart on your Battle List as they'll have purple names and a small tombstone beside them.

Unlike a normal Viking, they don't carry Gold and can't be hurt. Valhalla Vikings will only appear on your list when you've defeated all the regular Vikings, or there aren't many available. If your list is still full of Vikings for you to fight, the system won't generate Valhalla ones for you. Keep in mind you can only fight them so many times in a day before they melt back into the shadows and disappear, but not before you get your max daily Gamer Points!