Hey everyone! As my title says I want to dm for either family or friends depending on which happens first and I have an idea for a unique campaign that I think could be particularly fun I would love help on how to make homebrew stuff to make it work balancing and how to run interactions storytelling and portraying the narrative to the players in a fun way that let's them explore and make the story themselves too.
The main idea I have involves an npc recruiting a group of adventurers to take down these evil cultist leaders mob bosses and dark wizards who are trying to take over the city like an underground gang war and the city is trying to prevent them from taking over. Throughout this the npc that hired them would function as my way to talk to and guide the players being a mentor. They wouldn't participate in fights and would give guidance and help the party prepare but let the party run mostly independently. Their quest will be to capture the leaders to imprison them so the city can use the leader as leverage to keep the other crime groups at bay etc... during this the mentor character will be secretly killing the captured evil guys and taking a power of theirs through a dark magic and being led by an evil God who in exchange for the knowledge of how to take those powers and conquer the city for himself in exchange for letting the God out and the God then rules that area trying to take over the world. This campaign would involve a lot of heavy playing by me as the mentor turned evil npc that the party then has to vanquish at the end making a bitter sweet twist battle I hope where they fight someone who has all the powers of the bad guys they fought previously.
I don't want to take away from the players ability to enjoy and play the game they want to but I also want to tell them a fun story and give them a grand adventure with emotional highs and lows and have a satisfying and fun conclusion. How do I find this balance not overriding their free agency while also guiding them to that fun conclusion?
Similarly how should I go about balancing things giving the characters enough resources that they can reliably win the battles but it's not a cake walk?
Lastly how do I go about homebrewing the powers and magic systems in a way that leads to subtle foreshadowing and the twist while also not making it op and broken, something that fits the power scaling of dnd itself?