In the final act of the game, once you have the Ultimate Key from Havens Above, you can open all of those locked doors across Erdrea. Go to Cryptic Crypt, in the Hotto Region, and head all the way to the end of the dungeon. There will be four cells you can open with the Ultimate Key and the Tockle with the pastword is inside the eastern cell.
Upon arriving in Whealbrook, you will see that all of the NPCs are on the ground and if you speak with them, they will mention that they have caught some kind of cold. Head on over to the right side of the vilage and look for the one big house, entering inside and speaking to the NPC in the first room. Because of the sickness going around, he can’t do his job properly, but he has heard a rumor about a medicine that is able to cure any cold. The only hint you have to go on for the location of the medicine is Quester’s Rest, so that is your first step.
Speak to the green-clothed NPC behind the counter, Sellma, to learn that she can brew up some medicine for you, but she will need some drops of Yggdrasil Dew. It’s highly likely you already have some in your inventory, but if not, there are two easy places to get some. There is a Sparkly Spot on the path to Yggdrasil’s heart that yields one, plus you can also get one from the Puerto Valor Casino for 10,000 tokens. Once you have an Yggdrasil Dew, bring it back to Sellma and she will get to working on the medicine. You will have to exit and re-enter Quester’s Rest at this point to get the medicine from her, after which, you can bring it straight to Sancho in Whealbrook to finish the quest. Your reward is 5 chunks of Platinum Ore.