The heroes finally defeated the demon king.
Satan led the angels in rebellion against God.
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.
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.
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.