For the setting, I'm using the TSR's 1980 folio The World of Greyhawk. This is a slim 32 page book (not including maps) that provides a brief history, a gazetteer of political divisions, and descriptions of some of the major geographical features, and a couple pages on noble titles, knightly orders, and common runes and glyphs of the realm. In addition, I believe this book was the first appearance of the famed Darlene map of Greyhawk.
I wanted to start things out in a bit of a different situation from most conquer the borderlands games. So from the outset, the humanoid tribes would have the upper hand, the borderlands having been pillaged and sacked, and the player characters would be among a caravan of refugees fleeing those ravaged lands. During their journey, the caravan is attacked by goblin raiders lead by a great orc wearing fantastical armor. Other rumors speak of more of these powerful orcs and it seems they are the driving force behind the increased aggressions.
After perusing the gazetteer and talking to a couple people much more familiar with Greyhawk, I settled on the Barony of Ratik. This small country shares a hilly border with the Bone March, a war-torn land overrun with humanoid tribes. Ratik's eastern border is ocean and northern is the Frost Barbarian tribes, adding further inhospitable forces weighing in on the Barony. The western border is a mountain range called the Rakers. The Rakers run from the Flinty Hills, bordering the Bone March, north alongside Ratik before connecting to the Griff Mountains in the north. The mountains are both a natural defense for Ratik's western flank and a source of danger and adventure themselves, untamed and pocketed with many dungeons. It is in these mountains that I will find space for most of the dungeons I hope to run.
The first few modules I'm running, with mild spoilers, are: