。
Sorrow pierced his heart.
Literal
Sorrow [subject-が] his chest [into-に] pierced.
貫く (つらぬく) means 'to pierce, penetrate' — a vivid literary verb that personifies sorrow as a physical force striking the chest. The に-marked 胸 ('chest') receives the action. Common in narrative or poetic prose where strong emotion is rendered as bodily impact.