"It's...dust."
You had never considered how much could really change in your sleep.
You had been here before, you had walked through these woods for a time before meeting the skeleton. You had never seen this. You wish you never had.
The almost painful gleam of snow on the ground was gone, replaced with soot, ash, and dust, sitting atop the snow, making the door that much more difficult to open. It didn't pack like snow, instead coming up over your ankles, slowing your pace to lethargic walk. You wanted to believe that this was ordinary dust, ordinary ash, but the lack of cinder and white among the dark grey told you otherwise.
You walked slowly, like a person passing through a graveyard, because you were.
It continues. Past the door, all throughout Snowdin Forest. The silence is so complete it makes your ears ring, the crunch of your feet on snow barely audible beneath the blanket of grey. You come across a few ruined puzzles, including an invisible maze of all things. The walls are coated in the same powder, creating an odd effect over the area. From the distance, it looked as though the dust had exploded and been frozen in place.
The only thing that hasn't seen some form of decay is the bridge. You give the rope a few firm tugs, before realizing that your current alternative to crossing it is freezing to death.
"Nothing ventured..." you mutter warily, and cross. Even over the gaping chasm, the wind is absent. The air is still. It suddenly occurs to you that, in a cavern, that could only mean one thing.
Nothing is moving here...or anywhere. You, Toriel, and Flowey could very well be the only living things in the underground.
Hopefully the skeleton is around too. What happened here? I wasn't in the Ruins for that much longer.
That nightmare though.
Maybe the Ruins I fell asleep in weren't the same ones I woke up to.
You arrive in Snowdin, and receive no greeting for your trouble. The town is a mess. Everything else looked to have aged rapidly, but here...there was clearly fighting. Giant, charred holes are burnt clean through some of the houses. Nearly every window is shattered, and of all things bulletholes riddle the wood and stone. You run a finger over one of them, and it comes back coated in the same dust you're now up to your shins in.
Lots of other oddities surround the village. Giant, spiked pillars of ice ripping through the earth, sticking out above the dust in a dull blue shine. Entire carts overturned, fence posts ripped out of the ground and jammed into the sides of buildings. You follow the destruction until you reach a saloon, and are surprised to see it abruptly stop at the swinging doors.
Whatever did all this ripped through the town, making for the Ruins. They never went after anything further on. What were they after?
You continue your investigation, approaching the heap of cloth sitting in front of the door. What you find makes your heart sink.
A blue coat with a grey, layered hood. A single bullethole in the left side.
Its insides covered with dust.