What are demons?

Explanations, quotes, beliefs of different epochs and cultures, 
along with graphics, engravings and etchings , from different periods of art.
Daimonion (Pl. Daimonia): jewish/hellenistic 
Word (Septuaginta) for the different groups of evil spirits and ghosts. Amongst them are the Shedim, She'irim, and Sijjim, just as Lilith, Samael, Azazel, and Ashmodai. 

They bring plagues, harm and deceases of any kind, seduce to sin and spoil the soul. They cause deceases and destroy body and soul. They mess with personal belongings and bring need and misery. They act out their evil nature in houses, on fields, in desserts, unclean places, waterplaces and in forests. 

They act at dusk and dawn and during the night, and are mostly evoked through different spells, evokations and rituals that stand in direct connection with asceticism and renunciation.

Lexicon of Mythology

Demon threatens a magician (nearly 1850)

Belphegor (1863)
Quotations of greek Philosophers:

'All things are comply with gods and demons'
Epiktet (140 after Chr.)

'Demons are the inhabitants of a world netween the regions of gods and humans'
Plutarch (80 after Chr.)

"Demons' senses, instincts and affects are equal with the humans'"
Plotin (250 after Chr.)

"Wisdoms" of the middle age:

'Demons move faster than all birds'
(Yves de Cartres)

'Neither in heaven nor on earth, demons were and are only in the air'
(Hildebert von Mans)

'The demon is possible to work with supernatural forces only by plants'
(Rupert von Deutz)

'All demons are befalled on mental deficiency'
(Guillaume d'Auvergne)

Demon (G. Hellnw.)

Riding witches (Goya)
The Sabbath

''It is a phantastic view when those nightly figures come down from the clouds like ghosts. 

The devil as their lord in shape of a big, bearded goat, sits on a throne of gold surrounded by his demons, that appear in the bodies of terrible monsters with wild faces but also in those of the most beautifull and perfect humans. On Satans order the demons lead the present witches and magicians through the big fire that burns on the Sabbath-place... 

After the meal the couples of demons and humans dance in a round arm in arm in a weird formation that takes the most abstract forms and even makes the atrociousest grow pale'

(Delancre - Tableau de l'inconstandce de mauvais anges)

Demons are neither male nor female, they are just phantoms blown by the wind, like a storm ,who brings the dark right in the midlde of the day. 
The shadow of a demon is dark and there is no light on his body, he roams around at lonely places and left cities. 
The bands of demons are the sandstorm that flies over the skies. 
Demons are around the bed of a sick person and cause, that his body glows like in fire... 

(Textreferences from old-mesopotamic Scriptures)

Devils catch the souls (M. Wolgemut)