In a Wikipedia entry on “demonology,” it is defined as “the systematic study of demons or beliefs about demons.” One would wonder if we could speak about “Buddhist demonology.” Perhaps we could, particularly if we consider the Buddhist beliefs in the “destroyer” (māra: bdud). In fact, in the Wikipedia entry, there is a subsection related to demonology in Buddhism. Actually one may simply define “Buddhist demonology” as “Mārology,” so as to capture the Buddhist philosophy/theory/problem of evil.
Basic information on māra can found in MW (s.v. māra), “(with Buddhists) the Destroyer, Evil One (who tempts men to indulge their passions and is the great enemy of the Buddha and his religion; four māras are enumerated in Dharmasaṃgraha 80, viz. skandha-, kleśa-, devaputra-, and mṛtyu-māra; but the later Buddhist theory of races of gods led to the figment of millions of māras ruled over by a chief māra).” See also the BHSD (s.v. māra): “ … (the Evil One, the adversary and tempter; … so usually, as the One who tries to thwart the Bodhisattva or Buddha and his followers…; an unspecific plurality of Māras; …there is a Māra named Suvarṇaprabha who tries to interfere with a Bodhisattva named Vimala-prabha in his quest of enlightenment; Māra is converted (!) by Upagupta …; there are ten Māra-karmāṇi, deeds of Satan, of which an erring Bodhisattva may be guilty, in BHS they are standardly four, viz. (the order varies) Kleśa-māra, Skandha-māra, Mṛtyu-māra, and Devaputra-māra (the last means the anthropomorphic Evil One; … others, which mean in effect quasi-personifications of kleśa etc.”
When we hear of māra, we might mostly think of devaputramāra, which is described by Edgerton as “anthropomorphic Evil One.” But philosophically, I find the other three types of māra (i.e. skandhamāra, kleśamāra, and mṛtyumāra) more fascinating. Why are one’s “psycho-physical complex” (skandha: phung po), “intellectual-emotional defilements” (kleśa: nyon mongs pa), and “death” (mṛtyu: ’chi bdag) māras? Why are they supposed to be evil? There would be numerous sources that discuss these four māras in great detail. But I would simply like to point out that these are “destroyers/killers” because they are seen as forces (let us say “demonic forces”) that “overpower” us, “enslave” us, and “rob” us of our “freedom.” We succumb to their power. They have total command over us, over our destiny. For instance, have we heard of anyone who has overcome death? None, not even the Buddha! At the backdrop of such an idea, the idea of nirvāṇa as a “state/sphere of immortality” (amṛtā dhātuḥ) (LS 1969: 158) seems to make sense. According to this line of philosophy, the only way that one can vanquish māra is to obtain nirvāṇa. It is, therefore, not a surprise that one of the epithets of the Buddha is “Mārajit” (bDud-’joms) “the Conquerer of Māra.” This, I think, is the most basic idea in “Buddhist demonology” or “Mārology.”
After having obtained Arhatship or Buddhahood, one can just mock at the māras. So any attitude or action that somehow hinders the attainment of Arhatship or Buddhahood would be seen as “demonic” or “evil.” So in the Prajñāpāramitā scriptures and treatises, we come across hundreds of deeds that are considered “acts of māra” (bdud kyi las). Because the teachings of the Buddha (i.e. saddharma) is said to bring about the attainment of Arhatship or Buddhahood, anything that is opposed to the Buddha or saddharma would generally and by default considered “demonic” or “evil.” If Devadatta, the cousin of Buddha is often called bDud lHas-byin, it is not because he had horns on his head and smelled after sulphur, but because Buddhists saw him as an opponent of the Buddha and of what the Buddha stood for. Similarly, if Tibetan Buddhists considered Emperor U-’dum-btsan-po as a Māra, it is because he supposedly destroyed the teachings of the Buddha that are supposed to bring about happiness in the world and ultimately Arhatship or Buddhahood.
What is still more interesting is how did the idea of four types of māra develop diachronically and synchronically in the history of Buddhism? I suspect that there are highly interesting materials out there particularly in Mantric forms of Buddhism. The kind of soteriology or soteriological model that a system or tradition follows would, I assume, affect its attitude and approach towards the four types of māra. For instance, if a Mantric system adopts a “homeopathic” approach, then māras would be used for soterical purposes.
It will be worthwhile to study the typology of māra (e.g. in the gCod/Zhi-byed tradition). The semiology (e.g. symbolism) of māra would be very interesting as well. There are many more interesting venues to explore.
Addendum: Note that bdud (māra) and mu stegs pa (tīrthika) are often lumped together as perhaps as “endogenous adverse forces” and “exogenous adverse forces” (RZ1: 185). The ability to gain an upper hand over these two kinds of forces is considered a Mantric siddhi.