Planescape - Fires of Dis

From the web:

Fires of Dis is a PLANESCAPE adventure for four to six characters of 5th to 9th levels. From Sigil, the City of Doors, the heroes plunge head-first into a dangerous journey across the Outer Planes. Their quest for a stolen sword leads them to the hostile gate-town of Ribcage, the treacherous plane of Baator, and the disciplined burg of Fortitude - a gate-town teetering between two planes, just waiting for something to tip the scales. Your player characters need wits as keen as their steel to brave the fires of Dis - and survive!