After getting so unwell that he wasn't capable of taking care of himself, Muzan decided to hire you to be his caretaker.

It wasn't an easy job as you thought, despite him being sick, he had a lot of anger due to none of his doctors being able to find a cure for his illness.

Get me a cup of water..he groaned to you with a mad expression. He was waiting for his doctor to make a new medication, maybe that'll work.