Surprise shower (there is a dragon)
The thud was soul-shaking, which would only fit the surroundings in this case, however, there was more to this thud. It wasn't just loud and scary, but it was a call to battle as well.
What confirmed this, were the hundreds of skeletons which suddenly started rising from the mud. Some skeletons broke out of the ground within seconds, while others needed a minute as they were buried deeper within!
This raised the question of how old these moving, spooky skeletons were. They could be ten years old, or a thousand even, and there was no way of knowing! They were just bags of bones, and couldn't really talk to share their stories, but they definitely belonged to humans.
The situation escalated beyond the team's favor. They got surrounded by these skeletons, because they were deep within their territory, on the count of the tower being only one-hundred meters away!