Description:
The act of breaking into burial chambers, such as mounds, barrows, cists (coffins made of stone or hollowed out trees), tombs, etc., in order to retrieve treasure or special objects contained within. This is a well-known motif in the literature of medieval Iceland, played out both in legendary sagas such as Hervarar saga ok Heiðreks (The Saga of Hervör and Heidrek) and sagas of Icelanders, most prominently Grettis saga (the Saga of Grettir, one of the "outlaw sagas"). In this literature, the mound is often occupied by the living dead, who is in this case classified as a haugbui ("mound-dweller"), and who can be either cheerful, as in the case of Gunnar of Hlíðarendi, from Njáls saga; neutral, as is the case of Angantyr, the berserk and his eleven berserk brothers who were buried in the same mound, awoken by his daughter Hervör who asks him for the family sword, the cursed sword Tyrfingr, which he is reluctant to pass on to her; or hostile, as in the case of Kárr inn gamli ("the old"), whose mound Grettir breaks into at night in order to take the wealth and good sword that were buried with Kárr. Kárr awakens and grabs hold of Grettir, a struggle ensues, and Grettir beheads Kárr and escapes with his life, the treasure, and the sword.
Until more recently, mound-breaking was thought to be extremely rare in the archaeological record, and was mostly attested in the literature, but this has since been reappraised and found to be quite a common occurrence throughout Viking Age Scandinavia, and as far back as the Roman Iron Age. The reopening of burials and the removal of grave goods, long explained as mere grave robbing, has been theorized to represent a ritual transfer of power, particular in the removal of important objects such as jewelery and, more commonly, swords, which themselves are a symbol of military and elite authority. It has also been observed to affect a wider variety of graves other than the highest-status ship and mound burials. For an excellent article on the subject, see Alison Margaret Klevnäs (2016) "‘Imbued with the Essence of the Owner’: Personhood and Possessions in the Reopening and Reworking of Viking-Age Burials," European Journal of Archaeology, 19:3, 456-476. (LINK).
Mound-breaking in this sense can be immediately recognized by players as one of the most common occurrences in RPGs, where dungeon-delving takes characters underground in different settings, and quite commonly these settings are crypts, tombs, or grandiose necropolises. In these settings, undead are most commonly the kinds of enemies that players will encounter, drawing on famous fantasy scenes such as the Barrow-Downs and the Paths of the Dead from J.R.R. Tolkien's The Lord of the Rings books, as well as Peter Jackson's film adaptations of them, and other videogames' use of the trope and setting.
More recently, undead enemies in medieval fantasy RPGs have taken on a Nordic flair, being explicitly labeled as Draugar (the plural form of Draugr, the most well-known kind of medieval Scandinavian undead). This was popularized most widely through their appearance in The Elder Scrolls V: Skyrim, where the Ancient Nord cities-turned-crypts are full of treasure, traps, and Draugr.