الَّتِي تَطَّلِعُ عَلَى الْأَفْئِدَةِ
Which reaches into the hearts. (الهمزة: 7)
The relative pronoun allatī continues the description of the fire. The imperfect taṭṭaliʿu lends vivid immediacy to the image of its reach into the hearts, beyond a merely external effect.
This is a descriptive relative clause, not an oath, an exclusive restriction, or a fresh interrogative. It develops the answer already given about the fire and increases the force of the unfolding depiction.