。
An eerie silence made their blood run cold.
Literal
Eerie silence [subject-が] they [possessive-の] heart-liver [object-を] cold-made.
心胆を寒からしめる is a literary set phrase meaning 'to make someone's blood run cold' — literally 'to make the heart-liver cold.' 寒からしめる combines 寒い (classical attributive 寒から) + しめる (old causative, equivalent to させる). 無気味 (an alternate spelling of 不気味) = 'eerie.' Highly literary.