Cure to the White Death
Tuberculosis, also known as the White Death, was dubbed incurable until the 20th century. The treatment for the White Death was eradicating all the bacteria that cause the disease with antibiotics.
When it comes to completely erasing a mass population, the DeathDeathmonger of my Ultra Skill: Ruler of Death alone or the Unique Skill: Discriminator were more than enough to do the task. With just a snap of my fingers, viola, all the bacteria were gone, and the patient eventually recovered.
But I was not foolish enough to use my cards just to heal a single person from the disease. If I did and the words of my miraculous healing spread out, a person with an incurable disease would definitely crash our place and beg for it.
That was why I did not propose that idea in the first place. It would gather attention not only from the people but also from the church. I did not want to be hailed as a messiah or something that would end up with paganism.