I'm writing my first campaign ever. I told my friends that have run their own campaigns before that there's so much work for me to do that I don't know where to start. One of them suggested working backwards from the ending/BBEG, and I took his advice, but I'm already kind of stumped. The rest of this post is going to be lore-heavy, so please bear with me.
The campaign setting is Laerakond, a continent that was originally on Toril's twin planet, Abeir. It basically switched places with Maztica during the Spellplague. Since it was originally a part of Abeir, its history is greatly influenced by Primordials/Dawn Titans as well as dragons. In this case, the timeline I'm working with starts three years before the Second Sundering, when the overgod Ao works to once again separate the two worlds. There is a multitude of potential villains/powerful big bads that I could draw from within this setting, some of which are dead set on finding the "Shadow of Ao", a legendary artifact said to have the power to split the world in two. I've got somewhat of an idea of who I want the BBEG to be, but I have absolutely no idea what I would want the final confrontation to be. I'm drawing blanks as to what the nature of this legendary artifact is (is it even tangible?), and what work might go into finding or recreating it.
Am I going about this wrong? I can't even think of possible resources to draw from to try to come up with answer to the questions I have.
Ah! Welcome to the world of writing campaigns! For me, this is the most challenging, time consuming, and ultimately rewarding part of being a DM. Seeing your creation come to life, and players taking on dungeons, monsters and quests that you've devised is extremely satisfying.
I think you've got some good advice from your friends, and have built a solid campaign setting, with plenty of Lore and History for the players to sink their teeth into. Starting with the Big Bad, or ultimate conclusion to the campaign, allows you to slot in the pieces that are going to get them there, but don't plan out too many details at this stage.
Put together a framework for the chapters in your adventure. Start with broad strokes, and you can develop the details later. As you build your bigger picture, the smaller details will fall into place. Early quest will inform later quests, and help you develop your final encounter. A good place to start is 'The Heroes Journey' - which most adventures follow to at least some extent. Depending on the length of your campaign, you'll be able to take your players from early beginnings, to a gripping conclusion, with plenty of twists and turns along the way. It looks a bit like this:
1. Ordinary World
This is where your players enter the adventure, before the real tale unfolds. This is an establishing chapter, where your characters identify their true natures, capabilities and outlook on life. It anchors them to the world. This chapter is often short, and actually begins at character creation: It's the first night in the tavern. Or a quest to take care of a nearby cave of goblins that are harassing travellers. Or the journey to a nearby town. Or the humdrum patrol of a group of militia.
2. Call To Adventure
This is where the party receives some call to action. In the Lost Mines of Phandelver, it's the part where the heroes are interrupted on their journey by a goblin ambush, and track them to their hideout in Cragmaw Cavern. It could be returning from a quest to find the town under attack by a mysterious group of black-cloaked sorcerers. Or, in the case of Horde of the Dragon Queen, arriving at a city to find it under attack by a terrifying dragon. However it manifests itself, it ultimately presents a challenge or quest that must be undertaken.
3. Refusal Of The Call
In the Heroes Journey, this is the part where the hero has a crisis of self. Refusing to answer the call, second guessing themselves and their abilities. Personal doubts that prevent them forging forward. But it could also represent getting something wrong that reveals the vulnerability of the heroes. In Lost Mines, it's Sildar Hallwinter being killed by the goblins. But it could be arriving back home to late to stop marauding Orcs from razing the village. Or bursting into the wizard's summoning chamber too late to prevent the ritual being completed.
4. Meeting The Mentor
At this crucial point, the party is 'let in' to the wider story at play. In your campaign, it could be a wise mystic that knows of an ancient cult that's dedicated to discovering the Shadow of Ao and tasks the party in saving the world. It's side-quests that build a bigger picture of the threat at large. It's seeking out information, or a magical artefact that can help the heroes on their quest. Despite the earlier setback, this is the point in which the heroes vow (for whatever reason) to take on the adventure!
5. Crossing The Threshold
The party is now ready to truly begin the quest, in video games, this is 'the main quest', but in D&D, this is the overarching goal, that the party has at least some knowledge of. 'Save the Cheerleader, Save the World' or, 'Find the Shadow of Ao first'. This action signifies the parties commitment to the journey and whatever it may have in store.
6. Tests, Allies, Enemies
Now truly on the path, the heroes are confronted with an ever more difficult series of challenges that test him in a variety of ways. In LMoP, this is Chapter 3, where the players are tasked with gathering clues, leads and information about the location of the final dungeon. They help local residents, defeat a band of thugs terrorising the town, drive off a dragon, negotiate with a banshee, and free Gundren Rockseeker from the clutches of a goblin overlord. This is where there are exciting obstacles in the path; whether they're physical hurdles or people bent on thwarting their progress, the heroes must overcome each challenge he is presented with on the journey towards his ultimate goal.
The party needs to find out who can be trusted and who can't. They may earn allies and meet enemies who, each in their own way, help prepare the party for the greater ordeals yet to come. This is the stage where skills and powers are tested and every obstacle gives a deeper insight into the urgency and danger of the quest.
7. Approach To The Inmost Cave
The inmost cave may represent many things, such as an actual location in which lies a terrible danger, or an inner conflict which the party has not yet to face. As the party approaches the 'cave' they must make final preparations before taking the final leap.
This brief respite helps the party understand the magnitude of the ordeal that awaits, and escalates the tension in anticipation of his ultimate test.
Here's where your BBEG is faced - and hopefully the previous chapters have helped inform you as to what this should be. It should draw on all of the information the players have learned, combined with the overarching quest, and the lore of the world.
This is the high-point of the heroes story and where everything they hold dear is put on the line. If they fail, they will either die or life as they know it will never be the same again.
9. Reward (Seizing The Sword)
After defeating the enemy, the Reward may come in many forms: an object of great importance or power, such as the Shadow of Ao.
10. The Road Back
This stage in the Hero's journey represents a reverse echo of the Call to Adventure in which the party had to cross the first threshold. Now they must return home with the reward but this time the anticipation of danger is replaced with that of acclaim and perhaps vindication.
But the journey isn't over! In this final moment, the party must choose whether to fulfil selfish personal desires for power, wealth and glory - or sacrifice it all (destroy the Shadow of Ao!) to serve the greater good!
This is a really loose framework, but one I use for everything from one-shots to epic campaigns that span years. It doesn't have to be followed ridgidly, but you'll find that most adventures or stories you've ever read or played through follow it to some extent. Flesh out each part of the campaign, and continue to add detail until you're happy to start running the first part. What happens in that part might inform later chapters too.
Hope that helps a bit. If you just want some story ideas for your campaign in regards to the BBEG, I'll throw some out there!