Into the Mountains
Gundren set out with the party into the Sword Mountain's foothills early in the day. The party got the impression that he was going in routes that weren't very optimal, and he was stopping pretty frequently to check his map. The millitant among you were very concerned about the terrain as it didn't yield much cover from an ambush.
Gundren was very touchy about implications that he or the party was lost. He got super defensive when Stuu requested to take a look at the map and instead kept leading the party deeper into the mountains.
A day of travel through the rugged terrain passed and no enemies attacked you. Stuu had Droop start trailblazing about halfway through the day, discreetly, in an attempt to not lose your way on the way back.
At the end of the day Gundren declared that you had arrived. The cave in front of you had mine rails coming out of it and ominous sounding cave noises. Rather than go in unrested, the party opted to spend the night just inside the cave, after setting up a noisemaker.
During the night, Gundren seemed restless and he talked with Kronk about his life leading up to this point.
My brothers and I weren't very welcome at home when we left seeking our fortunes here. So I understand your unwillingness to talk about your past. That's all behind now anyway. You won't have to worry about your Aunt in Phandalin, once this mine is set up, that place is going to be rich.
He also had a talk with Torrfig:
How did you get mixed up in all of ... this? Surely you were welcome back home right?
Sorry I was so defensive about the map and this place. It's just... for all I know my brothers might be dead in there, and it doesn't feel right for me to share the sum of our lives work with anyone, even you cousin, without their ok.
If something were to happen to me. I want you to take the map. This mine will be worth a lot, and surely us dwarves will do it best right?
During the night, Droop heard something that agitated him. Something related to the goblin words for "meal" and "metal". Gundren told of a creature known as "Ironsbane" that eats metal often in old mines like this cave.
In the morning, the party ventured into the cave. There were no visible signs of the passing of Gundren's brothers, nor anyone else. And so the party crept forward. The unwavering light of the spell that illuminated Hada's sword drew many monsters to the party and they fought Stirges, something Torrfig recognized as a Rust Monster, and some freakish leathery things that wrapped themselves around their targets' heads while emanating darkness.
During the chaos of that last fight, the party realized that Gundren was gone. Previously he had been some 20ft behind the party, after the darkness subsided, the party looked around and could not find him. There were no obvious signs of a struggle, and no dragging marks were visible in that area of the tunnel.
The party did find some obviously burrowed tunnels they had overlooked before. Following them further into the cave, there still were no signs of where Gundren was. However, the party was soon accosted by the tunneler. A massive Umber Hulk burst through the wall and attempted to bedazzle the party with its mesmerizing eyes. After taking heavy injuries, the party eventually beat down this heavily armored foe.
Battered and confused, we left our party in the dark depths of this abandoned mine, concerned for their employer, and their own, health.