You need to create the medicine. You can for sure get the recipe from Othra and the scroll icon that you can find in your hut will give you a riddle on how to make it. (recommend to check that icon every once in a while as it has the recipes you may need in it.)
Heat Occisia-
1st: Angle leaf
2nd: Pulp (red)
3rd: Nothing
4th: Nothing
5th: Water lilly
Then you'll use this to make the cure, Spectral Sirup.
Spectral Sirup-
1st: Pulp (blue)
2nd: Heat occisia
3rd: Pulp (red)
4th: Heat occisia
5th: Pulp (blue)
Edit: Spelling