Let’s talk about Satan, because why not? The entity that became the Satan of the New Testament and Christianity as a whole has its basis in the Old Testament. Note: basis. But it didn’t exist there in anything resembling the form that it took in later centuries.
In the Old Testament, Satan is not a name like it tends to be used today. It’s just a word meaning “adversary”. In Hebrew, this is שָׂטָן and does not refer to this entity specifically. It is used multiple times thoughout the Old Testament to refer to multiple things. At one point, an angel of the lord was explicity referred to as שָׂטָן (Numbers 22:22). It seems like a general word for anything in the way.
So, it is not necessarily clear what the entity is, though it is implied that it has some relation to the בְּנֵי הָאֱלֹהִים, the sons of God (Job 1:6). The text does not specify exactly what it is, but makes it sound like a sort of cosmic pest in a celestial court, something under complete control of the Elohim. Everything it did, it did under orders from God.
So how did we get to where we are, where we have the figure of Satan as some powerful being fighting God to rule humanity? The intertesitimental period, also called the Second Temple Period. This was a time of great pain and suffering for the Jewish people, and one of the results of that suffering was something called the Testament of Job, a sort of rewriting on the Job that everyone knows from the Old Testament. What we see in this work is a much more malevolent שָׂטָן, one who delights in causing suffering and pain. This is more in line with what we get in the New Testament.
But where did that characterization come from? There are some theories that the Zoroastrian figure Angra Mainyu influenced the development of this character. Zoroastrianism is one of the most acient religions, and certainly one of the oldest still practiced. Similaries to Zoroastrianism can be seen in a number of places, but whether this is a result of direct influence or the simple fact that human cultures often develop along similar lines is debatable.