Oaths and curses figure strongly in the fantasy works of J.R.R. Tolkien because of their context within Old English / Old Norse speaking cultural traditions. An oath-breaker was a Nīþ, an extreme social outcast. Oath-breaking is the worst sort of violation and betrayal seen time and again in Tolkien’s works.
In the Silmarillion, the sons of Feanor vow a dark oath to reacquire the Silmarils at any cost, sworn in the name of Eru Ilúvatar, inviting against themselves a dark curse of everlasting darkness. Curses show up also in the case of Hurin and his descendants, however under the pronouncement of the greater evil of the First Age, Morgoth. Even Mim, the dwarf is able to curse treasure and others, but also following the violation of an oath of granted safety.
In the Lord of the Rings, the discussion of Elrond and Gimli on swearing oaths in support of the Fellowship is particularly interesting. Elrond councils against it – recognizing the power of oaths and how dire they can be. A group oath of the nine walkers of the Fellowship also invites a more terrible comparison with the the unbreakable oath of the men who accepted the Nine Rings of Power.
As a scion of a great and fated line of men, Isildur wasn’t necessarily a wizard. Yet he is also of a noble line that remained true to the teachings and oaths of his ancestors who, in turn, were also the descendants of the first king of Numenor – himself the brother of Elrond.
As a great lord himself, Isildur was bound by oaths of fealty to his father, and the receiver of oaths of many as rightful kind and heir. Although he was yet a man, even the lesser children of Eru Ilúvatar, elves, dwarves and men, are able to level a curse when an oath is violated.