"There are some different precise theological understandings. The story goes - very long ago, before humanity was created, there were angels, also created by God, and one of these angels decided that he wished to be more powerful than God. Wished to dethrone him and be worshipped as the greatest being in all creation, and to force all others to bend before his glory. I suppose perhaps it seemed less absurd at the time, having never been tried before. This angel amassed an army of others, who pledged to serve him, and not God, but they were found out, and there was a great war in heaven, between those who favored God and those who favored the angel. And ultimately the angel and his followers were cast out of heaven.
"Hell was created as a prison for them, a realm of fire and darkness. Perhaps it was created then. Perhaps it was created later, after we were, once the fallen angels - I suppose they are what you call devils - had become so twisted from their original purpose that they vowed to drag as many humans away from God as possible, and cause whatever suffering they could. It was they who introduced evil to us, and twisted our fundamental nature away from good. I'm not sure what else is said about them. I'm not very permitted to study my own scriptures, you see. But that's what Hell is. Hell is where one goes if one persists in being twisted toward evil, if one refuses the offer to return to God and be forgiven. At the end of time it will be a prison to lock away evil, and there will be no more suffering outside it."