In the garden of eden the devil told eve that she would not die when you ate from the tree of the knowledge of good and evil.Which was a lie.But were in the bible does it say the devil made up the idea of the immortal soul?
It doesn't. By telling her that she would not die, when God said that they would, death came early when their firstborn son murdered his brother in a fit of jealousy.
Adam lived for 930 years, so physical death was not immediate but from the day they ate the fruit, they were separated from God, experiencing spiritual death. Physical death would occur after God had allowed them to fulfill his mandate to "fill the earth" with their kind.
When they experienced death, it went against their 'programming' so in time, when false religious ideas started to creep in, a continuation of life in some invisible realm, seemed like the only way to make death acceptable. Since all false worship carries this idea, and all false worship is satan's idea, then life after death simply perpetuates his first lie, making it appear as if God was the liar, not himself.