Arimanius (Greek: Αρειμάνιος; Latin: Arīmanius) is a name for an obscure deity found in a few Greek literary texts and five Latin inscriptions. In Greek texts, Areimanios, with variations, seems to refer to the Persian Ahriman, in the context of "Zoroastrianism" as interpreted by the Greeks and Romans.
The most extended passage in classical literature on Areimanios is two sections within Plutarch's treatise Isis and Osiris, who describes him as the dark or evil side in a dualistic opposition with Oromazes (for Ohrmuzd or Ahura Mazda). Areimanios is also mentioned in other texts as an evil daimon, "the worst spirit", or even equated with Satan as "the adversary".
The most common form of the name in its few occurrences among Greek authors is Ἀρειμανιος (Areimanios), presumably rendering an unattested Old Persian form *ahramanyu, which, however, would have yielded a Middle Persian ahrmen. The name is given as Ἀριμάνης (Arimanēs) by Agathias, and Ἀρειμανής (Areimanēs) by Hesychius, rendering the Middle Persian Ahreman. Variations of the name not explicable by systematic linguistics may be attributed to its similarity to Greek words meaning "warlike"  (see Names and epithets of Ares).
According to Plutarch, 46, 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 Hades ("The Hidden One") and darkness. (In Greek religion, Hades was the ruler of the dead or shades, but not per se a god of evil, except in the sense that death might be considered a bad thing – κακόν, kakon.)
The Areimanios ritual required an otherwise-unknown 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. Plutarch, 47.
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 doubted  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. 
As a Mithraic godEdit
Franz Cumont was the proponent of a now-unfashionable view that Greco-Roman Mithraism had been influenced by some beliefs of ancient Mazdaism, including ethical dualism. Most scholars doubt that Mithraists preserved the doctrine of Persian magi, despite appeals to their authority, but the name Arimanius is difficult to divorce from the Persian tradition of Ahriman. At the same time, the five high-quality dedications to Arimanius found throughout the Roman Empire fail to suggest that he was conceived of as an evil being in a Mithraic context, however formidable; Gordon remarks: "the real point is surely that we know nothing of any importance about Western Areimanius."
No evidence of a spot in a mithraeum for the "omomi" cult described by Plutarch has been found, and the association of Mithras and an evil god has been dismissed by some scholars as inherently implausible. The inscription Deo Areimanio ("to the God Areimanius") is found on a few altars to Mithras, without any description that would link him with a particular iconography. Occultist D. J. Cooper conjectures that the lion-headed figure does not represent a god, but rather shows the spiritual state achieved in Mithraism's "adept" level – the Leo degree.
A mutilated statue at York 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 Mithraism 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.
- Beck, R. (1984). Mithraism since Franz Cumont [in] Aufstieg und Niedergang der römischen Welt II.17.4 .
- Beck (1984, p. 2034)
- Gordon, R.L. (1975). Cumont and the Doctrine of Mithraism, [in] Mithraic Studies. Manchester University Press.
- Gordon (1975, p. 226).
- Plutarch. (1936) [tr] Thayer, W. Isis and Osiris, 46–47. Loeb Classical Library
- translation by Bill Thayer available at LacusCurtius 46–47.
- de Jong, A. (1997) Traditions of the Magi: Zoroastrianism in Greek and Latin Literature. Brill.
- Diogenes Laërtius 1.8; Damascius, Dubitationes et Solutiones 125; Agathias, Historiae 2.25; Theodore of Mopsuestia apud Photius, Bibliotheca 72.81, as cited by de Jong (1997, pp. 313–314) In the Mithraic context, the name seems unlikely to refer to an evil entity. Gordon (1975, pp. 226–227).
- de Jong (1997, pp. 312–313).
- Reinach, S. (1909). [tr] Simmonds, F. Orpheus: A General History of Religions. London: Heinemann.
- Turcan, R. (1992, 2001). The Cults of the Roman Empire. Blackwell; originally published 1989 in French.
- Salomon Reinach (1909, p. 68) gives the identification as Pluto, the name of the Greek ruler of the underworld used most commonly in texts and inscriptions pertaining to the mystery religions, and in Greek dramatists and philosophers of Athens in the Classical period. Robert Turcan (1992, p. 232) notes that Plutarch makes of Areimanios "a sort of tenebrous Pluto". Plutarch, however, names the Greek god as Hades, not the name Plouton used in the Eleusinian tradition. For distinctions in usage between the two names, see Pluto in the mysteries and cult and Pluto in Greek literature and philosophy.
- Boyce, M. & Grenet, F. (1991). A History of Zoroastrianism: Zoroastrianism under Macedonian and Roman Rule, pp. 458–459. Brill.
- de Jong (1997, p. 313).
- de Jong (1997, p. 314).
- Turcan, (1992, p. 232).
- Gordon (1975, pp. 217 ff).
- Gordon (1975, pp. 226–227) lists the dedications as CIMRM 322 (Ostia); 369 (Rome); 833 (York); 1773, 1775 (Carnuntum).
- Gordon (1975, p. 227).
- Bivar, A.D.H. (1971). Mithra and Mesopotamia [in] Mithraic Studies, p. 278. University of Manchester.
- For instance, CIL III.3415, III.3480, VI.47 (without naming Mithras);
- D. Jason Cooper (1996). Mithras: Mysteries and initiation rediscovered. York Beach, Maine: Samuel Weiser, Inc. pp. 48–49.
The statue is a representation of the Leo degree, internalized.
- Turcan (1992, p. 232).
- Beck (1984), pp. 2034–2035.