The Story So Far.......
The White Valley of Slaughter is named after a battle long ago to protect something in caves within the valley. A minotaur hero killed during the battle was buried to continue protecting the caves. Giant chalk figures on the sides of the valley are maintained by descendants of the victors, nearby villagers. The chalk figures include a bull and a minotaur among others.
Answers for the Questions
Summoning our ideas we look at possibilities for the remaining questions
Q. Who were the enemies at the battle?
With nothing obvious we can make it other humans, either another people or continue the bull theme with a cult that sought out the secret in the caves. Or we could go for the original inhabitants of the caves, other minotaurs or spirits associated with the chalk figures.
Q. What was being protected in the battle?
It has to be something worth fighting a battle for within the caves, so an item of great power, a place of significance in the caves such as a shrine, earlier burial ground or portal.
Q. What's Behind the Vast Metal Door?
This is the obvious place to put what is being guarded by the hero, but it could also be a treasure of the caves or a spirit or ancient being locked away. the original description points towards some sort of fire magic defending it.
Q. Why is the wraith there and why does it seek release?
The obvious argument for the wraith is to be the hero, but it could also be related to the thing the hero is protecting. It could be another defender of the tomb or unrelated to the tomb. The hero would seek release if cursed in some way, but the thing being guarded would be a more obvious candidate for seeking release and having a reason to communicate with those entering the caves.
Q. What happened between the fallen skeletons in the dungeon?
The giant skeleton must be the buried hero, so a minotaur skeleton with either humanoid allies that fell protecting the tomb, or the skeletons of those who were slain by the hero protecting the tomb. Maybe they were villagers looting the tomb or part of a bull cult.
The Ancient Cult of the Bull
Drawing inspiration from the D&D 5E minotaur entry and minotaur skeleton I like the idea of a hidden cult among the villagers worshiping a bull-like demonic lord. This gives us the foes in the battle being bull cultists, the fallen skeletons being later cultists from the villages around and the thing being protected something they want.
This gives us bull cultists as Adversaries, the hidden cult as a Mystery among the surrounding villages and Plot Hooks tying PCs to the bull cult as their background, enemies or religious rivals. We've also worked on our History for the adventure.
Next up the thing behind the metal door is something the cult is after, so an ancient shrine holding a spirit of the cult fits. This could then be manifesting as the wraith which manages to continue the cult over the years even if purged from the villages.
The wraith gives us an NPC to interact with, and answers the Secret Mystery of the Caves and gives possible Treasures within the shrine should the PCs loot it.
Lastly the minotaur skeleton of the hero still protects the tomb, and in the past could have been an outcast from the cult standing against it. This provides a possible Adversary, Ally or source of Rewards to the PCS depending on how they approach it.
So we've answered the last of the questions and given us a framework to fit in more elements as we generate rooms and their contents....