After the landlord died, his three children paid him tribute with three cigarettes, which eventually ended up in the hands of Han Fei.
Each of these cigarettes corresponded to one of the landlord's children's lingering thoughts; by lighting up the cigarette, it was possible to awaken the residual consciousness of the person, and to gain their protection before the smoke dissipated.
When Han Fei encountered Zhuang Ren in reality, he had entered the old house where the Chairman of Eternal Life Pharmaceutical once lived and accidentally discovered a broken music box, a wooden puppet wearing a mask, and a piece of children's clothing smeared with paint that bore words and blessings given to one's own child.
Within the music box, there remained a song for soul-summoning, its melody almost identical to the Deep World's song.
From this, Han Fei deduced that the song from the Deep World might be one of the landlord's three children.