Viking practice of
dueling for goods, wealth and marriage-rights (or just women-as-plunder).
Banned in 1015 by King Erik along with public fits of berserkergang for not altogether unrelated reasons - if the berserker went into the fit (as was likely) before the duel, he would be nigh-invincible. The man so challenged would have to choose between certain death at the hands of this blood-crazed madman or being named nidingr.