Satan; the Devil; the Prince of Darkness
Refers to the supreme evil being in Christianity, often used in fantasy contexts as well.
魔王は天使たちを率いて神に反逆した。
Satan led the angels in rebellion against God.
king of demons (Buddhism)
In Buddhism, the king of demons who obstructs good deeds; often equated with 天魔 (Tenma).
See also: 天魔
The heroes finally defeated the demon king.
In that story, the hero sets out on a journey to defeat the Demon Lord.
In Buddhism, the demon king is considered a being that obstructs the practitioner's enlightenment.
多くの魔物が魔王に仕えている。
Many demons serve the demon king.
The seal broke, and the demon king was revived.
天魔 refers specifically to the Buddhist demon king, whereas 魔王 can also mean the Christian Devil or a generic fantasy demon lord.
サタン is the proper name for the Christian Devil, while 魔王 is the descriptive title 'demon king' that can refer to Satan or other rulers of demons.
天使 is a benevolent heavenly being, the direct opposite of 魔王, the supreme embodiment of evil.
魔法 means 'magic' or 'sorcery,' while 魔王 means 'demon king'; they share the kanji 魔 but refer to different things.
From 魔 (ma, 'demon') + 王 (ō, 'king'). The term is used in both Buddhist and Christian contexts, as well as in modern fantasy.