Dagor Dagorath

Why didn't the Valar imprison Morgoth in the end in the Halls of Mandos from where nobody (nor Valar) can escape? Why did they send him into the void from where he escapes in the end for the Dagor Dagorath, when they could keep him in the Halls of Mandos instead and chill?

Hey guys just a random question that came to my mind. Have you ever thought about the Dagor Dagorath as the last battle of a depressed or menthal ill person with him or herself? The different ages would show the different moods that person was in and no one knows what the battle will be like and if they're really in it when it takes place. No one will also know if this is really the last battle. If it ends good well it ends good and if not well that could mean suizid? Let me hear your thoughts about it :)

I've had an idea lately to write some fanfiction covering the very end of Middle-earth (the Dagor Dagorath) and right before then, but will definitely need to create a lot of new characters and some new places since I don't know what exact time period it will be in. However, I am no great expert on the languages and don't want any names to be inaccurate (one of things I'm going to try to do is write this story as if it could actually be canon and with a similar writing style as LOTR); does anyone know of a website or something where I can translate things to the languages (e.g. Quenya, Sindarin, etc.)? It would be very useful. :)