。
Eventually, the ghost disappeared into the thick fog.
Literal
Before-long ghost [topic-は] thick fog [possessive-の] inside [locative-に] disappeared.
A literary sentence. 軈て (やがて) is the kanji rendering of the adverb meaning 'before long, eventually, soon' — nowadays almost always written as just やがて in hiragana; the kanji version (軈て) is considered archaic/literary. 幽霊 (ゆうれい) is 'ghost'. 濃い ('thick, dense, dark'). 霧 ('fog, mist'). The sentence has a classic storybook rhythm.