The devil is believed to be a very powerful, supernatural deity that is evil and the enemy of gods and humans. Apep was the enemy of the gods and humans. Set was considered to be evil. However, Apep was nothing like Set. Apep was always the evil force–not a force to reckon with. Apep was a giant snake, and he wanted to get rid Ra ( the sun god) forever. It was believed that Apep was associated with the norther sky. The northern sky was a dark place and very dangerous.
Every time Ra traveled into the underworld, Apep tried to kill, but Set and Mehen would always protect the sun god from all danger while they traveled across the sky at night or into the underworld. The plan was to cut Apep’s belly to help Ra escape from Apep. If Set or Mahen didn’t protect the god Ra, the world would be over and everything would fall into darkness.
Apep was a very large snake. Funerary texts inside the tomb of Ramesses VI have paintings of Apep. Twelve heads were painted above the head of the snake to represents the souls he ate. The souls were said to be free when he was destroyed, but when the night came, Apep would eat the souls again.
In many versions of the story, it was Hathor or Ra turning into cats and carrying with them a knife to kill the big snake, and not Set.
Apep was not worshiped like the other gods. Well, every religion has some rebels, and perhaps Apep was like Satan– worshipped without your parents knowing. There’s no doubt that he was feared, and he was also a very powerful deity because he CANNOT be destroyed. He would live forever.
Credits: http://socyberty.com/history/the-history-of-the-evil-god-apep/#ixzz1cZy23JMq