。
She vanished into the darkness.
Literal
She [topic-は] darkness [of-の] inside [into-に] figure [object-を] erased.
姿を消す ('to vanish, disappear from sight') is a fixed idiom — literally 'erase one's figure.' Stylistically dramatic, common in fiction and journalism describing fugitives, ghosts, or someone who slipped away unseen. 暗闇の中に gives the destination of the disappearance — に on 中 marks it as the goal/location of vanishing. The whole sentence has a cinematic, almost noir flavor.