Diary found on the skeleton of a tiefling, found in the Lower Caverns.
This “diary” is—or, at least, starts out as—more of a travel journal, recording various mundane details of this man’s travels through the wasteland: plants and animals observed, weather conditions, interesting or unusual rock formations and terrain features, etc.
The journal was clearly intended for its author’s own purposes (though what those purposes might’ve been—cartographic? scientific? other?—is unclear), so there isn’t any introduction, explanation of who the author was, where he came from, what he was doing, etc.; he does not even record his name. He seems to have been an explorer of some sort, and clearly had no small knowledge of the natural world—the entries betray a keen expert’s eye for flora, fauna, and other natural phenomena.
The journal is written in Common. The language is is a bit odd—“formal” would be an apt description, but it’s more than that; the characterization that comes to mind is that it reads like it was written by someone who’d read a lot of old books. You gather that this tiefling came from Donaille, just as you did; although, from the recorded observations, it seems like he took a somewhat different route through the wastes than you (at one point he mentions passing within a few miles of a mountain; this perhaps suggests that his journey took him a good deal further north than yours has).
Toward the end is where it gets interesting. It seems that the tiefling came across a goblin settlement (whether this is the same one that you visited, or a different place, is unclear). He managed to talk his way into being a guest of the goblins for a time. While there, he apparently stole a magical amulet belonging to one of the goblin leaders (this, it seems, would be the oddly adorned rat skull you found in his pack).
It’s unclear from his account whether the tiefling intended to steal this amulet all along, or whether this is something he decided to do on the spur of the moment. In any case, the theft was discovered, with predictable consequences; the tiefling was forced to flee. He managed to escape from the goblins’ lair, but they pursued him across the badlands; his flight eventually brought him here, where he sought shelter in the caverns. He notes that the goblins at first pursued him even into the caverns, but soon gave up and left. (These last several entries, where he lays out his account of the events that brought him here, were evidently written while he was hiding in the caverns, waiting for the goblins to finally give up and leave entirely.)
The last major entry includes the tiefling’s musings on the amulet he stole. He evidently believed that he’d made a significant discovery: “Even as our own magic continues to wane,” he wrote, “it’s clear that these wretched creatures have found sources of power, out here in the wastes, that permits them, even now, the creation of enchanted items! Their magical creations are weak, rudimentary… but the fact that they are still able to imbue stable items at all is itself a revelation, and contrary to the common belief that such things are no longer possible. What I have seen in my travels through this nearly-dead land suggests that it’s no coincidence that I’ve found such a thing here, and not in the east. If I manage to survive this fiasco, I expect that my discovery will motivate more than a few expeditions into these wastes…”
The diary ends with a brief note that scrabbling, squeaking, skittering noises could be heard from the nearby tunnels.