、。
Night fell and, worse, it started to rain.
Literal
Night [to-に] became, moreover bad thing [at-には] rain [subj-が] began-to-fall.
さらに ('moreover, even more') + 悪いことには ('to make matters worse,' lit. 'on the bad thing') is a set written phrase for adding negative conditions. The fixed idiom ~ことには is used for 'much to one's X' constructions — 驚いたことには ('to (my) surprise'), 悲しいことには ('sadly').