((Alright! The dungeon shall begin right...NOW!))
The travelers would suddenly feel their pack, if they had one, become slightly heavier. If they checked inside, they'd see a runic piece of parchment with a seal on it. Attached to it was a sticky note (Copyright McIna Industries) with a slanted handwriting on it; A town portal scroll. Break the seal in a moment of peace to be instantly teleported back to the sign post. Then, the mists in front of the dungeon parted slightly, revealing the entrance to the cave.
After entering, the explorers would see a magma-like cave, with out-of-place cobblestone occasionally worked into the floor at odd angles and a small lava drip coming off of obsidian stalactites, which was mostly harmless; if you were lucky, it just scalded you slightly. If you weren't, it would solidify mid-air and hit you on the head. Most of the time, you're unlucky.
The tunnel extended off into the distance. Scrawled into the wall, there was a massive, single number: 1.