A One Round Living City Adventure
From the introduction:
The Maztican continent has, for most of its history, been undisturbed by lower planar entities, seeking to use native populations to further goals concerning the ancient Blood War between the tanar’ri and baatezu. Of course, there are exceptions. Centuries ago, a powerful ultroloth accidentally arrived in Maztica. Aware that it had stumbled across unexploited territory, the fiend began appraising the land.