The Tomb of the Serpent Kings. Everyone knows the lands around here were ruled by a snake-man empire in ancient times, but a goat-herd came back saying that she'd found one of their tombs. For most people, that was a warning.
For you? Riches and glory glittered in your eyes.
It'll be dangerous, which is why you've brought along allies. And a few of the local folk to watch your horses and maybe carry things. Before you lies a tunnel leading into the hillside, the long grass that obscured it pulled away.