Horkos ( ancient Greek Ὅρκος, " the oath ", Latinized Orcus, Latin Iusiurandum ) In Greek mythology, the personification of the binding force of an oath.

In Hesiod's Theogony Horkos is a descendant of the Goddess of Discord Eris, his siblings are the personifications ponos ( hardship ), Lethe ( oblivion ), Limos (hunger ), Algea (pain), Hysminai ( battle), Makhai ( fight ), Phonoi ( murder), Androktasiai ( slaughter ), Neikea ( Hader ), Pseudea ( lie), Amphillogiai ( verbal dispute ), Dysnomia ( lawlessness ) and Ate ( delusion ). He brings greater destruction upon the people than any of his siblings, when they have become a perjurer. In the works and days he pursued judges who make false judgments of corruption, in addition to consecrate him the Furies, the goddesses of vengeance.

In Hyginus Mythographus Iusiurandum is descendant of Aether and Terra.

Aisopos told in a fable of a man who is a fiduciary receives money from a friend, but plans to keep the money for themselves. On the way out of town he meets a lame man, who introduces himself as Horkos and says that he visits each city only once every thirty to forty years. The next day the man puts an oath on managing money well. When he met Horkos it and this pushes him to the edge of a cliff, he asks him how it can be that he met him again the next day. Horkos tells him that he would come back also on the same day, if he provokes it. The moral of the fable is that there is no predictable time for when the perjurer of Horkos is punished.

Since the word ὅρκος originally referred to the object to which an oath is filed, Horkos is also a term of the Styx, the Greek gods put on the water of their oaths.