。
The tyrant's heart was softened by feelings of pity.
Literal
Tyrant [possessive-の] heart [topic-は] pity [possessive-の] feeling [with-で] softened.
哀れみの情 is an elegant pairing: 哀れみ (pity, compassion) + 情 (feeling/emotion), linked by の. 和らいだ (past of 和らぐ, to soften/ease) is a literary verb more at home in prose than conversation. で marks pity as the instrument of the softening. The sentence has a classical, storybook quality.