الَّذِي يُوَسْوِسُ فِي صُدُورِ النَّاسِ
Who whispers into the breasts of humankind. (114:5)
Alladhī is a relative pronoun and yuwaswisu is the verb in its relative clause; the understood subject points back to the relative pronoun. The construction further identifies the activity of al-waswās al-khannās. Classical grammarians offer more than one case analysis for the relative expression.
The rhetorical effect is to move from a named threatening agent to the recognizable act by which its threat operates.