Once you have the Quarantomb Key, you can go with Dr. Phlegming to investigate the curse afflicting Coffinwell.
Enter the Quarantomb
Go west from Coffinwell to find the Quarantomb. It is in the north part of the Western Coffinwell area.
In the Quarantomb, talk to Dr. Phlegming. Afterward, go north through the hole in the wall.
Quarantomb - L1
You might sometimes see Metal Slimes here. Be sure to fight them. They are very likely to run away before you can defeat them, but if you do manage to defeat them, they give a large amount of experience.
Go east from the entrance and open the red chest in the southeast corner to get a Slipweed. Then go north and open the red chest to get a Mini Medal. Then go north and check on the statue to press the switch. Then go west and open the red chest to get a Seed of Skill. Then go west and check on the statue to press the switch.
After you have pressed the switch on each statue, the chamber in the center will unlock.
Go to the center chamber and check on the jar. The Ragin' Contagion will attack.
The Ragin' Contagion can attack twice in a turn. Sometimes he will attack two different people, and sometimes he will attack one person twice. He also knows Kasap and Deceratle. These decrease defense and agility.
Once the Ragin' Contagion is gone, you will get some Royal Soil, and Dr. Phlegming will be able to seal the contagion away again. Dr. Phlegming will go to the chamber below, where the amnesiac ghost of a king will be wandering, and Dr. Phlegming will be reading the tablet in the center of the room.
Back in Coffinwell
Return to Coffinwell and talk to the Mayor. Then go and talk to Catarrhina. There will be a cutscene. Next, if it's not night time, rest in the inn until evening. Then talk to the Mayor in his house and you will get a Feather Headband.
While it's still night time (see above), go to the church and talk to the ghost on the left. Then go check on Phlegming's door (under the stairs in the northwest part of town). After some dialogue, the doctor will join you. Go around and talk to everybody. After a while, Phlegming will say that he has seen enough. Go back to his place and there will be a cutscene.
Back to the Starflight Express
Your next stop is the Starflight Express. It's in the Mountain Pass, west of Stornway and east of Angel Falls. When you go inside, check on the controls and you will return to the Observatory.
You can go around and talk to everybody if you wish. There is an optional cutscene in the room on the left side of L1. Then go up to the top of the Observatory and pray to Yggdrasil. There will be a cutscene. Then you will learn Zoom, which lets you warp to any town that you have been to before.
Exit the area and there will be another cutscene. Now go down one flight of stairs and go clockwise to reach the engine of the Starflight Express. Go through the door of the train and there will be a cutscene. Afterward, talk to Stella. You will head down to the Protectorate and end up on Newid Isle.
You can go to Alltrades Abbey to the northeast. That is your next destination.