The Great Stair has been called such since the first elf was sent down into Infra many millenia past. It is a combination of the exit of a magical portal originally created and tended to by the elves of the surface and thousands of steps carved from a massive conglomeration of stalagmites grown together over the passage of time.
The Stair has, in the time since the settling of the Drow, changed hands from elfin to human and the small Drow fort and town that had been carved into the stone of Infra is now inhabited and decorated by humans. The architecture reflects the efforts of the Drow in their first years in the World-Beneath-the-World.
The Great Stair is the only viable means of transporting people and supplies to or from Infra. The Serfs deliver regular batches of supplies via the Stair for their prisoners in the world below; each nation of serfs supplies a percentage of their largest exports for shipment to Infra.