。
The princess begged the emperor for clemency.
Literal
Princess [topic-は] emperor [to-に] clemency [object-を] pleaded.
姫君 (ひめぎみ, 'princess') is the traditional respectful form — 姫 + 君 (honorific). 皇帝 (こうてい, 'emperor') is the Chinese/generic emperor (distinct from 天皇, the Japanese emperor). 寛恕 (かんじょ, 'clemency, magnanimity') is classical/literary vocabulary. 請う (こう, 'to beg, to request') is formal. The whole sentence reads like historical fiction or a fairy tale.