Get on the Enterprise and head south. You will be in the town Agart. There should be a
well there. Select the Magma Stone item.

A cutscene will show.

Get on the Enterprise and enter the hole. (Press B Button when you're above the hole)

Then after you will have a cutscene again and you are stuck to the Underworld. Enter the
Dwarven Castle and enter the Throne Room. From that point you will fight Calca and Brina
(Calcabrina). After that battle, you will fight Golbez.

The rest you must find out.

next go save the game go throu the door the sisters guarded then
go throu the next door get rosa and figh wind arch fiend then
use teleport

he already beaten the Arch Fiend of Wind and rescued Rosa. He just needed to know where to
go next after those two events.