、。
Unfortunately, he broke his leg in that accident.
Literal
Unluckily [even-にも], he [topic-は] that accident [by-で] leg [object-を] ended-up-breaking.
不運にも functions as a sentence adverb meaning 'unfortunately' — に adverbializes the na-adjective 不運, and も adds emphasis. てしまった marks the action as completed with regret. 脚 specifically means 'leg (the limb)' as opposed to the more general 足 which can mean foot or leg.