。
She's possessed by a demon.
Literal
She [topic-は] demon [by-に] is-being-possessed.
悪魔 ('demon, devil') leans more Christian/Western in connotation than 悪霊 ('evil spirit'), 鬼 (Japanese ogre/demon), or 妖怪 (yokai, Japanese supernatural creatures). Supernatural possession in Japanese is conventionally rendered as a passive: 取りつかれる puts the host in the recipient role and the spirit as the agent marked with に. ~ている marks the ongoing possessed state.