The cornucopia (also known as horn of plenty) is a symbol of abundance and nourishment, commonly a large horn-shaped container overflowing with produce, flowers, nuts, other edibles, or wealth in some form.
The cornucopia became the attribute of several Greek and Roman deities, particularly those associated with the harvest, prosperity, or spiritual abundance, such as personifications of Earth; the nymph Maia, and Fortuna, the goddess of luck, who had the power to grant prosperity.
Jason Grace and Piper McLean are sent on a quest by Hercules to retrieve Achelous' other horn to humiliate him even further, since Hercules had cut off his first one. When Jason and Piper get to his river, Achelous is singing. Piper thinks that the singing is has the same effects as charmspeaking. Achelous assumes that the demigods are there to kill him at the request of Hercules. He explains to them why Hercules did not deserve his horn, and tells them the story of Deianira. Although both Piper and Jason feel bad for Achelous, who seems to be friendly, the god tries to stop them by drowning them. While Jason is pulled under, Piper tries to bargain with Achelous, but the god believes that she wants to be his bride, to compensate for Deianira. When Achelous picks her up, she threatens him with her knife to let Jason go. Piper then cuts off the god's horn and leaps for the shore, using Jason's power over the winds to carry her safely to the bank. Achelous wails as he sinks back into the river to mourn his horn.
Hercules never gets Achelous' horn, because Piper forces him to give them permission for their ship to leave. Piper uses the horn as a cornucopia at Hercules, and food comes pouring out of it, burying the god. The Argo II flees from Hercules' island, leaving behind a mourning Achelous and an angry Hercules. Piper then uses it when she, Jason Grace, and Percy Jackson are drowning in the well, and then again when they are battling the giants. She ties a string to it and wears it like she would a purse. It is used in later books as one of Piper's weapons.
While fighting Mimas in the Temple of Fear, Piper McLean offers the cornucopia as a sacrifice to her half-brothers: Phobos and Deimos. She uses it to slice off the head of the statue of Ares, releasing the makhai within.
- The cornucopia is apparently able to create food using the thoughts or feelings from anyone in the area, not just the person holding it. This is shown when Piper uses the cornucopia for a picnic and a birthday cake pops out, despite Jason never telling her when his birthday was.
- The cornucopia's role is vaguely similar to the role of the Nemean Lion's Pelt. Both were obtained by defeating someone, are associated with Hercules, used by the one who defeat it's wearer, are animal parts, and are eventually given up. The differences are the cornucopia is more of an offense as opposed to the pelt being defense and Piper had the cornucopia much longer having it for three books as apposed to Percy Jackson who only had the pelt for one.
- The Latin translation of Cornucopia is 'Horn of Plenty', stemming etymologically from 'Cornus', meaning 'horn', and 'copier', meaning 'plenty'.