From Webtoon/TappyToon:

Erkin, a young pharmacist from the north, has grown up orphaned by the war between the Northern Nations and Mormeratta. After making a name for himself in the newly formed Kingdom of Mormeratta, he is suddenly summoned to treat a mysterious “master of the castle” whom no one has ever seen. His lack of knowledge of both this sickness and the patient he’ll be treating leaves him hesitant to accept the offer, but he reluctantly agrees in hopes that it will bring him one step closer to finding the infamous Witch of the Sun who killed his parents during the war. Little does he know, on his quest for vengeance, the witch may have been by his side all along. What will Erkin do when confronted by his sworn enemy turned friend?
---From uploader iLuv2Procratinate