Debilitating Elixir.
As the name implies, this elixir leaves one weak for a period of time. Not only that, due to the weakened body and decreased immunity, it is very easy to keep attracting various minor illnesses, and the unlucky may even develop serious diseases.
The elixir is not difficult to concoct, and does not take too long either; it can be done in half an hour. When the time comes, I'll just need Shitou to cover for me.
Lu Yun thought that since the Su family was still so lively, it must be because they were in excellent health, with too much energy they had nowhere to vent. In that case, it would be better to let them be weak for a while, calm down, and take their medicine nicely, so they would stop coming out to seek attention and annoy others.