Wikipedia
Heitstrenging, also known as Hietstrenja, Heitstrengingar or Strengdir, was the Norse ritual of making solemn vows, particularly at Yule. The practice was eventually discarded when Christianity gained dominance in the region.
The vows were traditionally made while laying hands on the bristles of the sonargöltr, the boar sacrificed at the blót on the evening of Yule Eve, and in association with the Bragafull or chieftain's toast. According to some sagas, the speaker would stand and first place his foot upon a stone or bench, prefacing his vow with the words "I mount on the block and solemnly swear...".
The association with the sacrificial boar and the ritual toast gave the vows the force of an oath. However, due to the ritual process, they usually came after the speaker was drunk. They became a topos in the later sagas, often as a form of bragging and promising the performance of some great feat, and taking place not only at Yule but at other sacrificial feasts, weddings, arvals or just ordinary banquets. The vows were solemn and there were punishments for breaking them, but they were not always important. Harald Hairfair, the unifier of Norway, promised to not comb or cut his hair until he ruled the entire country. More often, though, they were regarded as a matter of great significance. In Jómsvíkinga saga, the compatriots of Sweyn Forkbeard vow to ravage Norway and kill Haakon Jarl, while making sure to rape Thorkill's daughter.