So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him. — Revelations 12:9
The word Devil (diabolos in Greek) means 'the slanderer' or more literally 'the one who divides.' The word Satan means 'the adversary.' These are equated with the serpent from the Garden of Eden, who is (for the purposes of a fantasy RPG setting) is wonderfully called the great dragon. In Luke 10:18 Christ tells us that, "I saw Satan fall like lightening from heaven." In Isaiah 14:12, Satan is called the Day Star ("Lucifer" in Latin) which proceeds the rising of the sun. This takes on significance when put in context of Christ's title the Sun of Righteousness.
Given all of these images, the Devil was a being of high importance (usually understood as an archangel). He rebels against God by reaching for godhood:
'I will ascend into heaven; I will place my throne above the stars of heaven. I will sit on a lofty mountain, on the lofty mountain to the north. I will ascend above the clouds; I will be like the Most High.' — Isaiah 14: 13-14
In this rebellion he takes with him a group of angels. In doing so, he aligns himself with the nothingness from which creation comes. He becomes the personification of nothing. His actions try to pull creation away from God and back to the nothing from whence it came.
Then the serpent said to the woman, "You shall not die by death. For God knows in the day you eat from it your eyes will be opened, and you will be like gods, knowing good and evil." Genesis 3:4-5
As the serpent, Satan deceives Adam and Eve and gets them to follow in his sin of reaching for Godhood. When Eve gets her husband to eat of the tree of knowledge of good and evil, she and Adam sought knowledge. Specifically, they wanted the knowledge of evil — a world without God.
Thus, when creating a fantasy world, there needs to be an Adversary — a personification of Chaos bent on pulling the world away from God. Given the number of names associated with this being in Scripture, we are free name him according to the context of our world (translate "adversary" or "slanderer" into some version of elvish, for example). For purposes of world bulding, it is important to know that the name Adam means humanity. This story invites us to use it metaphorically. A fanstasy world does not need an Adam and an Eve per se; however it does need humanity to seek a world without God — the knowledge of evil. This pursuit is the result of a deception by the Adversary. To add flavor, one can use the metaphor suggested in Gensis 3:3. Eve tries to defend God by adding to the command of God to not eat of the tree of Knowledge of good and evil. She says to touch it is to die. Thus, the Adversary can use the good intentions of humanity to lead them to their fall — in trying to defend God, they come to know a world without God.