1. The origins of the name itself
The 6th-century Agathias gives the name as Ἀριμάνης. 5th-century Hesychius gives the name as Ἀρειμανής. These both seem to render some Middle Iranian derivative of Avestan 'Angra Mainyu', perhaps Middle Persian "Ahriman" as attested in the 9th/10th-century works of Zoroastrian tradition. The older Greek authors give the name instead as Ἀρειμανιος, the -yu form of which either renders some other older unattested derivative of the Avestan name, but could also have emerged naturally if the Greek translators were attracted by the corresponding Greek word for "warlike".3 The few Latin instances of the name presumably derive from the Greek versions.
2. In Greek literature
The most extended passage in classical literature on Arimanius is found in the treatise Isis and Osiris (46-47) by Plutarch, who presents him as the dark or evil side in a dualistic opposition with Oromazes (for Ohrmuzd or Ahura Mazda).4 He is also mentioned in other texts as an evil daimon, "the worst spirit," or even equated with Satan as the adversary.5
According to Plutarch,6 Zoroaster named Areimanios as one of the two rivals who were the artificers of good and evil. In terms of sense perception, Oromazes was to be compared with light, and Areimanios to darkness and ignorance; between these was Mithras the Mediator. Areimanios received offerings that pertained to apotropaism and mourning.
In describing a ritual to Areimanios, Plutarch says the god was invoked as Hades7 ("The Hidden One") and Darkness. (In Greek religion, Hades was the ruler of the dead or shades, and not a god of evil, except in the sense that death might be considered kakon, a bad thing.) The ritual required a plant that Plutarch calls omomi, which was to be pounded in a mortar and mixed with the blood of a sacrificed wolf. The substance was then carried to a place "where the sun never shines," and cast therein. He adds that "water-rats" belong to this god, and therefore proficient rat-killers are fortunate men.
Plutarch then gives a cosmogonical myth:
Oromazes, born from the purest light, and Areimanius, born from darkness, are constantly at war with each other; and Oromazes created six gods, the first of Good Thought, the second of Truth, the third of Order, and, of the rest, one of Wisdom, one of Wealth, and one the Artificer of Pleasure in what is Honourable. But Areimanius created rivals, as it were, equal to these in number. Then Oromazes enlarged himself to thrice his former size, and removed himself as far distant from the Sun as the Sun is distant from the Earth, and adorned the heavens with stars. One star he set there before all others as a guardian and watchman, the Dog-star. Twenty-four other gods he created and placed in an egg. But those created by Areimanius, who were equal in number to the others, pierced through the egg and made their way inside; hence evils are now combined with good. But a destined time shall come when it is decreed that Areimanius, engaged in bringing on pestilence and famine, shall by these be utterly annihilated and shall disappear; and then shall the earth become a level plain, and there shall be one manner of life and one form of government for a blessed people who shall all speak one tongue.8
Mary Boyce asserted that the passage shows a "fairly accurate" knowledge of basic Zoroastrianism.9
In his Life of Themistocles, Plutarch has the Persian king invoke Areimanios by name, asking the god to cause the king's enemies to behave in such a way as to drive away their own best men. It has been doubted10 that a Persian king would pray to the god of evil, particularly in public. According to Plutarch, the king then made a sacrifice and got drunk, a virtual motif of how Persian kings act in Plutarch, and thus dubious evidence for actual behavior.11
3. In Latin inscriptions
Several altars to Mithras are inscribed "Deo Areimanio" (to the God Areimanius) and a statue of the 'lion-headed personage' in the museum at York (CIMRM 833) seems to have the name of Areimanius on it (but see below).12 The altars were published by Franz Cumont.13 The first of these is a dedication to the god in fulfilment of a vow, but does not refer to Mithras by name.14 The second is two altar bases, one to Mithras and one to Areimanius.15
Richard Gordon states that there are five high-quality dedications to Arimanius found in the Roman Empire.16
Reinhold Merkelbach gives the following list of inscriptions which give the name of (deus) Arimanius:17
No evidence of a location for the omomi cult described by Plutarch has been found in a mithraeum, and the association of Mithras and an evil god has been dismissed by some scholars as inherently implausible.19
Cumont believed that the word Areimanius was a Romanised version of the Zoroastrian Ahriman, the evil principle in dualism, and used this as evidence that Mithraism was of Persian origin. He therefore identified the leontocephaline god in Mithraeums with Ahriman. But Turcan states that "hardly anyone now subscribes to Cumont's idea that Graeco-Roman Mithraism inherited certain beliefs of ancient Mazdaism" and suggests instead that the lion-headed image represents the divine fire, not a Pluto-like deity as given in Plutarch De Iside.20 R. L. Gordon says that the meaning of these inscriptions and dedications is not actually known and "the real point is surely that we know nothing of any importance about Western Areimanius."21
4. Roman Britain
A mutilated statue at York, CIMRM 833, has a fragmentary dedicatory inscription that has been read as containing the name Arimanius. The figure seems to be entwined with a serpent, and at one time it was conjectured that it represented the lion-headed god of Mithraism22 or a form of the Mithraic Aion. But since Arimanius can also be a personal name, it is uncertain whether it refers in the inscription to the god represented by the statue, or to the person who made the votive dedication. No other Mithraic objects were uncovered near the statue, and any leonine features are subject to fancy.23
5. Other examples