Guarding the Shrine

In ancient times, there were many dragons across these mountains in the north. As little folk spread across the land, and discovered the wealth under the ground, they contested the dragons and drove most of them from the skies. In due course, cults of dragon followers arose and established shrines and strongholds to worship the dragons and train their own warriors to avenge the wrongs brought against the dragons.
One of these cults carved a shrine into the heart of the mountain across the valley from the abandoned outpost. The outpost was set up to keep watch on the valley after the cult was destroyed, specifically to guard access to the cave holding the shrine and make sure the cult doesn't rise again. Unfortunately, a kobold tribe found their way into the cave system from deeper within the mountain. They began worshipping at it, drawing in more supporters. Too late, the watchers noticed something was going on there, and they were attacked and defeated.
One man, Everett, managed to survive the initial assault, and hid in the basement. When you find him, he is unconscious and barely alive. If you stabilize his wounds or heal him and allow him to take a long rest, he will recover completely. Once he has, he can relate further information.
The watchers drove out the original cult decades ago, and have maintained a watch over its location since. One of the key elements of the shrine was a powerful magic item. When the cult was destroyed, the item was removed and safeguarded by the watchers. They concealed it, and it was not found when the building was attacked and overrun.
Everett will recover the artifact if the players haven't found it already, and request they help him take it safely to another location. He knows a powerful wizard that lives in a tower a few miles upriver, and thinks the wizard can help unlock the power of the item, and offer advice on what to do with it now.

Overview

What I have in mind is an adventure that starts at the coast somewhere and follows the river up into Aron's Bowl. My original thought was that there could be a back door into the mountain reached by crossing the Molten Hills. They are rank and foul, but offer an unexpected route to the peak. It is unguarded in an official sense, but inhabited, making it guarded in a territorial sense.
This is a good location for a clan of slaad. I think the trip up the river is one that encounters mountain creatures, bandits, lairs of kobolds and goblins and similar monsters. There are also good opportunities for remote outposts, towers and retreats of cultists and ranger types with their own secret agendas.
My plan is to build an adventure that can be started at 1st level, and gradually built up to a level that is sufficient to face a full grown dragon. This is preliminary, but I think it should be pretty easy to get a party to fifth level by the time the river turns north. This gives opportunity to face far more daunting foes whether they follow the river through the heart of the mountains, or decide to cut off the big loop and head directly across the exposed peaks. Either way, I think you're looking at one or two major lairs of orcs or something similar, and encounters with giants of whatever kind you prefer. The yellow areas are fully mountainous areas, with the brown areas representing the extreme peaks of the Iron Teeth.
In the plateau around Aron's Bowl, the main contacts are with miners. The upper part of the river leading to it is home to many mines. These are constantly tapping tunnel systems that provoke attack from underground creatures. The activity and extracted wealth is also drawing interest from everything in the area. Travel can be difficult, and include encounters with pretty much any level of monster.
The difficulty is going to be narrowing down which monsters to use, etc. Story-wise, I want to have a powerful magic item discovered early on, in the abandoned outpost at the edge of the mountains. The outpost was set up to guard and research the item, but also to keep tabs on a group of cultists in league with kobolds. The cultists have found the outpost and decided it is time to act. They attacked and ransacked the outpost without finding the item they sought. The PCs find it, but have to defend themselves against a follow up assault. This sets up the dynamic of dragon connected cultists that can follow, shadow or intercept the party at different points along their journey.
I feel like at this point I need to break the adventure up into more manageable parts, and plan them by level. I think this will work nicely. My concern is that with a partly civilized area in the middle, the adventure could flatten out too much. I think three tiers will work nicely, with the first getting the party to fifth level, the second taking them to eighth, and the third to tenth. Then the dragon lair itself as a separate encounter (or set of encounters).

Abandoned Outpost

Arriving at a small clearing high on the hillside, you have found a building with attached stable and gated yard. Unfortunately, the gate has been knocked off, as has the front door of the main building. The northern wall of the building is dominated by a towering second story with slits opening in the middle of its upper wall. From the far side of the building there is smoke rising. It doesn't appear to be on fire, but the smoke is rising from the entire length of the building, indicating recent trouble.
The whole place seems deserted, though a large fight clearly took place. There are signs of horses running off down the trail, chased by much smaller feet (Survival DC12 will recognize them as kobold prints). A thorough search of the main building will show many bodies, kobold and men, and a few goblins as well. The back half of the building is badly burned, with gaping holes in the rear wall and roof. Looking through them shows the whole river valley laid out below the clearing, clearly an advantage of this location.
In a large room to the left of the entrance, there is a staircase spiraling up into the tower. The upper floor is scattered with discarded and broken weapons. There are arrow slits in each wall giving coverage to the surrounding area. In the next room on the first floor there is a stairway leading into the basement.
Investigation DC15 will reveal that there were several potions and scrolls in the room at the back of the house, mostly consumed in the fire. If the check is 18 or higher, you recognize that the fire was set  intentionally to destroy the contents of this room. Searching (Perception DC15) will show three potions that survived the fire among the many smashed flasks (2 potions of healing and a potion of speed). DC18 will let them find three scrolls that escaped destruction (Alarm, Fog Cloud, and Sleep).
The rest of the first floor is scattered with broken items of all sorts. It appears that a dozen guards were stationed at the garrison. There are two larger rooms, one with sparse furnishings, the other with scribbled notes and arcane ingredients. If players look for a specific weapon or component, roll a percentile dice and give them a 10% chance of finding it among the wreckage.
Downstairs in the basement, little has been disturbed. There is dust and cobwebs everywhere, but no light sources. There are a number of small rooms carved out of the earth.
Perception DC12 to find a secret trap door in one room. In the room is an unconscious guard. He can be healed, but remains unconscious until he has had a long rest. Searching the sub basement can reveal a buried chest (DC18) with a mysterious orb inside.
While searching the building, the sun sets, leaving you to set up camp in the burned out building for the night. It does offer good shelter and protection, but it is also the target of an attack in the night. The kobold gang that ransacked it returned to their master with news of the victory, only to be savagely bullied for failing to recover the orb they were sent to find. They are sent back and attack in the night. There are 8 kobolds, 4 goblins, and 2 winged kobolds. There are many ways to play out the encounter, depending on where the players make their camp, what kind of watch they set, etc.

Where there's smoke, there's fire...

Traveling southeast from Denner's Bend toward Pinching, the mountain trail follows the contours of the ridges up and over the slopes of the higher peaks. The trail skirts the high valleys, hugging the outer spurs, running through forests of pine and birch.
Now the trail is turning down toward the moor, where it crosses to join with Black Ridge Trail which runs from Pinching along the Black Root Forest north of the River Flushing. You've been sent from Kellen to seek information on the dragons of the Iron Teeth. The clerics there have become concerned with stories from merchants and tradesmen about increased dragon activity along the ports and down the coast. Few outside the monastery know, but one of the young acolytes has been experiencing visions of dragon attacks, and they are concerned that the attacks are nothing to what lies ahead. They have told you of their concerns, and sent you forth to discover what you can.
For the last three days you have crossed the mountains separating Kellen from the coastal plain. Much discussion has passed among you about where to go, what course to take, but no consensus has been decided upon. Events are approaching to spur a decision.
Cresting the final ridge and looking down into the valley and across the moor, you can see Black Ridge Trail coming out of the forested hillside below. Looking back up the valley from where the trail emerges, you can see smoke rising from a point higher up on the ridge in the forest. There is too much smoke for a campfire or fireplace, but nothing else in the area that would cause so much smoke.
You quickly decide to follow the trail towards the smoke, in order to find the cause and assist in any way you can. You find a small path leading off the main trail up the hillside to where the smoke is rising. As you move carefully forward, you notice a clearing at the end of the path. There is a fenced yard surrounding a small one-story building with an attached stable. The gate is broken open, as is the front door of the building. The smoke is rising from the back side of the house which all seems to be smoldering.
What lies in the ruins? Where will this encounter lead?

Dragon Talk

"Some, I suppose, but the most powerful is Antaraxes Glavenis, and he lives at the highest point, Scarred Peak. Legend names him the father of all reds, but it is more likely a title. There is always conflict among dragons, and invariably the young surpass the old in strength. The old is defeated, and the young claims the lair of the ancient, becoming Antaraxes Glavenis as well. I have seen him once, enormous and terrible. Wings a hundred feet across, or so it seemed. Thankfully, I survived to be here today, selling you what you need.” The dwarf was tiring of the history lessons, and noticing other customers making their way around the town square, was eager to make a sale and talk to more people.

 There have long been rumors that Scarred Peak is laced with huge caverns and snaking passages, formed out of hollow lava tubes and enlarged by dragons and their willing slaves. Not surprisingly, there are no actual first-hand accounts to back up such tales. Occasionally, residents of the hills and mining towns, even the coastal towns that cling to the towering cliffs that butt the Iron Teeth and Scarred Peak, witness great winged creatures flying in or out of the mountains gaping crown. Whether it is one dragon or many, it presence is definite. And the hills are crawling with kobolds, the most likely fawners to serve a dragon.
Still, though dragons have been seen, they are rarely spotted abroad. In fact, the most recent sighting was more than ten years ago. No one really believed the dragon had died, but all looked at the lack of activity on Scarred Peak as a promising sign. Seismic activity was negligible, and prosperity grew among the prospectors and speculators in Aron's Bowl. There was a sense that fortune might shine on them all for years to come.
But three months ago, the earth had been shaken in the dark of night with a terrible fury. Since then, frequent earthquakes have been felt, and an almost constant smoking has been rising out of the many cracks and openings into Scarred Peak. There is concern that Antaraxes Glavenis is stirring, and will soon sweep down across the land wreaking havoc as it hasn't seen in over a hundred years.
More modern and practical folk have looked at the history of the mountains, considered all the legends and tales passed on from mother to son, from pub to pub, and from sailor to sailor in every port, coming to the conclusion that Antaraxes Glavenis is a title more than a name. They believe the increased activity portends doom for many, but not because the dragon is waking, but because its slumber and dotage has been challenged by the conquering virility of youth. A new, strong, young dragon has come to the mountain and is waging battle for its wealth.
One way or the other, all agree that terrible times are approaching. Word and coin have begun passing through the land, calling on any brave enough to make their way into the smoldering crevices of the peak and rid the mountain or one dragon or all. Promise of wealth beyond measure has stirred the sinews of many adventurers, and stoked the embers of greed in so many more. There is fame to be won on the peak, if one can survive.

Moor and Mountain

The land around Pinching rises steeply to a barren highland that runs for 20 miles from the coast to the mountains. This moor is full of steep dells and bowls that break up its otherwise flat appearance. These give access to numerous cave systems, which have attracted all manner of bandits. Little happens on these moors by day, but under the cover of darkness, it crawls with activity.
This is a bleak area of the world, stuck in the middle of hotbeds of activity. The Iron Teeth are an ominous barrier, and only marginally accessible, yet they are a doorway to riches through their mines. The sea is a ready conduit for cargo and men, but the great moor lies bleakly between. It is a region that must be crossed and often, and yet is so porous and concealing that it is far to dangerous to cross safely. It is a mix that factors into any campaign.
The mountains themselves form a wall along much of the southern coast of the moor, with only the narrow valleys carved by rivers over time allowing access into them. Yet the dominate feature of them is the mountain towering over them all, Scarred Peak. In all likelihood, the entire region is part of a giant super volcano, with the hot springs and thermal pools of Aron's Bowl the caldera of the giant. Of the ring surrounding it, Scarred Peak sits atop the highest slopes as a vulture on its high perch, threatening all in its shadow.
From the west and the north, the walls of the Iron Teeth rise so sharply and steeply that no passage can be found to the Peak. From the plain of Aron's Bowl, the slopes step broadly up toward the high peaks in dangerously inviting fashion. The slag from the weeping scars of lava giving the peak its name creep down these steps, mingling with the waters feeding the Flushing River, to create a hazy, sulfurous quagmire above the plain known as the Molten Hills.
The river itself starts high on the slopes of the southern face of Scarred Peak, snaking south and east around much of the Molten Hills, then turning north to cut through them and drop onto the plain of Aron's Bowl, then on to the sea. The eastern mountains of the ring can be scaled fairly easily, as well as those to the south. Their danger lies in their wild, untamed ways, and the extreme conditions that pervade the north once you get away from the thermal vents of Scarred Peak and Aron's Bowl.
The approach to Scarred Peak from the east and south is gentle by comparison to most of the other mountains of the range. This invites ascent, which risks the wrath of the dragon lairing within the volcano. Mostly dormant, the great wyrm within has become active of late, and begun reigning terror down upon the whole region, mountain, moor, and sea.
Tomorrow, we will consider rumors of the great beast itself.

Driscoll Vale

Pinching is a town at the mouth of the Flushing River. The Flushing cuts a gorge out of the moors running along the southern coast of Glacier Bay. There are many roads running out from Pinching to the north and south toward other settlements along the coast. The river gradually rises onto the moor and across it toward the east. As the river reaches the wall of mountains, it winds its way up into them through a deep valley cut by the water through long years of erosion. This is Driscoll Vale.
This is a hotbed area of activity for all sorts. Traders and miners use it to travel between the coast and Aron's Bowl, a broad plain high in the mountains cradled within the surrounding, towering peaks of the high mountains and Scarred Ring. This is an area of volcanic slag running down from the restless volcanoes in the high peaks.
Driscoll Vale, particularly near the moor, is rich in caves. These caves harbor all manner of creatures - from goblins to drow. This combination of travelers and monsters make the vale a common place for encounters. There is a great danger of kidnapping, banditry and general ambushes while traveling the trails up into Aron's Bowl. Most parties travel in groups for safety.
There are increasing rumors of organized activity by goblins and bandits kidnapping people to force into the mines as workers. Some believe they are being sold into slavery with the drow, others think the goblins themselves are beginning to exploit the mineral wealth of the hills and capturing people to do the work for them.
All that is know for sure is that Driscoll Vale has become a difficult stretch of road to pass through, and people have been disappearing from caravans and local towns and farms.