A Hospital
Hell energy was quite versatile while also being volatile for most humans. The people in Lucius's past world could resist it using various means, such as mana, healing spells, magical tools, natural resistances, and many more.
In this world, there was no such thing as magic, and thus mana did not exist either. But what did exist was Aether which was preset in nearly every person in this world along with the myriad of creatures.
Lucius himself had seen the reaction between Hell energy and Aether in his body. But he knew he was a big exception as he had the hell imprint to control it, along with having a soul that was not from this world.
He wanted to see its effects on an average human of this world. And by average, Lucius meant physically and not financially. Keith was the son of Viscount Drew and was nothing but average for most people, but his physical condition was not that different from any commoners.