Seth (Sutekh) — God of Disorder & Defender of the Sun
Not the Egyptian devil: the violent god who murdered Osiris was also the only one strong enough to kill the serpent of chaos.
Also: Seth · Set · Sutekh · Setekh · Sutech · god of chaos · god of the desert · Seth animal · murderer of Osiris · defender of the sun-barque
Egyptians worshipped Seth as the god of the desert, storms, violence, strength, and foreign lands — a turbulent power, but emphatically not a simple 'devil.' In the Osiris myth he is the murderer of his brother and the rival of Horus; yet in the solar cult he stands in the prow of the sun god's boat, spear in hand, as the one strong enough to kill the chaos-serpent Apophis each night. Some kings even bore his name with pride. Only later, especially in the Greco-Roman period, was Seth increasingly recast as a wholly evil being — a demonization that distorts his older, ambivalent role.
In short
Seth (Sutekh) — god of disorder, storms, the desert, and foreign lands; an ambivalent rather than simply 'evil' deity, both the murderer of Osiris and the indispensable defender of the sun-barque against the chaos-serpent Apophis. His later flattening into a devil-figure is a Greco-Roman-era demonization, not the older Egyptian view.
Where it’s found
Early Dynastic royal monuments; the Osiris and Horus-and-Seth cycles; New Kingdom solar texts (Seth defending the barque)
How the idea developed across Egypt
Attested from the Early Dynastic Period (Seth animal on royal monuments; a 'Seth-Peribsen' royal name in Dyn. 2); honored in the Ramesside period (kings named Seti, 'Man of Seth'; Sutekh as a state god). His progressive demonization into a near-Satanic figure intensifies in the Late and especially Greco-Roman periods — a late development, not the original conception.
Seth is a deity (never a person). Explicitly resist the 'Seth = evil/devil' shorthand: in pharaonic religion he is ambivalent (both Osiris's murderer and the sun's defender). The wholly-evil characterization is a late, partly Greco-Roman demonization — present it as such, not as the Egyptian baseline.