Molech, Legion and Abaddon are demons named in the Bible, and Azazel, Beelzebub and Kokabiel are demons named in the Apocrypha. The idea of demons comes from Jewish folklore stories about fallen angels, including Satan.
Biblical scholars describe demons as evil angels based on the idea that when Satan rebelled against God, he took a third of all the angels in heaven with him. Jesus describes hell as an eternal fire for the devil and his angels in Matthew 25:41.
The Bible names Molech in the book of Leviticus 18:41 and 1 Kings 11:27, but never directly states that he is a demon. The verse in Leviticus warns the Israelites not to allow their children to worship Molech or they would offend their own god. The verse in 1 Kings calls Molech "the abomination of the Ammonites." Molech worship involved human sacrifice, and this practice continued until the Israelites became captives in Babylon.
Legion is the cumulative name of many demons possessing a man together as described in Mark 5:2-10. The possessed man was so strong that no one could restrain him. Jesus spoke to the demons, and they stated that they were together called Legion and their numbers were many.
Abbadon appears in the Bible during John's Seven Trumpet's prophesy in Revelation 9:11. Abbadon appears when the fifth trumpet blows as the leader of the demons that torture those who have not accepted Jesus as their savior.