1 - The missing villagers of Tar

Even for those how are not experts in undergrounds, a visible pattern is emerging as you travel through this abandoned mine: rooms of approximately the same similar size succeed to each other, opened in every cardinal direction unless the copper vein told otherwise. There's something reassuring to this symmetry, a charm that shouldn't not leave Sedror insensitive.
Remains of cart track show that there were three ways out of this room and a small, weak point of light appears at the end of the westmost path.
You are now standing in what looks like a dried riverbed that appears to go south toward the entrance of the mine and east into some unknown but somewhat similar cave. In the shallow riverbed the ground is hard and friable, a property that Tianni put to good use as she looks for tracks using her low light vision.
All the tracks that Tianni finds are rather recent, no more than a couple of days. She isn't comfortable enough with the anatomy of the spider creatures that inhabit this mine to tell exactly what they were, but she can clearly see that creatures seem to be coming and going from the east and to the outside. Whatever lies behind the massive doors that close the path that goes east in the mine's entrance has probably been left untouched for ages, as no track seem to go that way.

Overview and closeup look maps

After taking a minute to recover from the messy fight you just had, you advance in the eastern tunnel. Mee'haïl is holding his torch (that is almost entirely consumed), while the rest of you enjoy their low light vision. Unlike what you saw so far, the tunnel doesn't go directly into another room : instead, it turns south and continues on something like a hundred feet. The tunnel has been dug in hard stone and show no sign of having any trace of copper. Either they followed a thin vein and ended up with that odd-looking tunnel, or they were simply searching for another one.
Anyway, the tunnel gets larger and larger as you approach its the next room. As you follow the track you can see that they continue up the walls and disappear in three or four large openings in the ceiling just before the end of the tunnel. These holes stand at about twenty feet above the ground.
The odd tunnel transitions into a natural cave that have been arranged as an important-looking hub. But this is not what's notable about this room : what is notable is that there are very explicit signs of battle here. Even if you can barely see half of the room, it's impossible to miss the dwarven skeletons that just lie on the ground in their shredded work clothes or damaged armors. Busted shields and old-looking weapons are scattered around the place.
There are pieces of wood everywhere, and clear signs that a massive explosion occurred here: a large crater
stand in the side of the "hill" close to the door, partially collapsed in its center, and menacing cracks around the crater testify to the strength of the explosion.
Most of the dead dwarves in work cloth are gathered before the closed doors that you can see on your right (to the west). The armed dwarves' bodies are closer to where you are standing, or just down the central elevation. It seems like those who ended here faced two fronts of enemies.

Tianni take a few cautious steps to get a better look at the scene.
The graveyard-like place gives some details up. Every body has been reduced to its simplest form, leaving only skeletons or piles of broken bones. The air is still, immobile even, in that place that appears to have stayed untouched for years. You can't precisely tell how long ago the massacre took place, but it looks like you where not born yet when it happened.

Sedror, your knowledge of stonework reveals that many stone block have been moved recently from the tunnel where you are standing and rolled or thrown into the natural cave. Traces on the ground and white marks on the stones that would be gone if it happened more than a few days ago.

Mee'haïl, you'll have to go check them to determine that. Do you? It'll be a simple Wisdom check, so that's 21.
What you already know is that if they were made of common iron they'll probably be very damaged. But as this appears to be a copper mine, it might be some kind of more durable alloy.

As Mee'haïl walks in the room, unpleasant noises can be heard. Sinister cracks echo in the cave.
He gets in the room and start to look at the equipment on the dead dwarves. It seems like they fought with what they could find : pikes, wood axes, work hammers and metal bars... But some of them though, mainly those that are equipped like soldiers, seem to carry strange equipment. Upon closer look, it looks like it is made of gold, but is way harder than the expensive metal. They even seem harder than your average equipment, although somewhat heavier. There are warhammers and battleaxes, three handaxes and a shortsword. You discern what looks like a small round shield under one of the bodies and two of them have a helmet on their head, whether the head is still attached to their bodies or not.

As Tianni is reaching for the shield, you hear the whistling of something flying toward you two. You duck by reflex but receive a large, sticky web that glues you both to the ground. The second after, a giant spider and one of the spider-kobolds land on you ready to eat your faces.

But even with the advantage of their ambush, things don't go as well as they planned. The moment they land you hear a frightening and loud crack coming from the ground. The cracks near the crater lengthen at incredible speed, crossing the whole room and reaching the walls. The ground cracks and escapes under your feet while fear grips you.
Everything falls apart: the ground, walls, dwarven skeletons... Mee'haïl's torch gets extinguished as you all fall in complete darkness.