Atmosphere reversed in unhappy destiny (4)
I believe the answer is:
doom
'unhappy destiny' is the definition.
(prepare to meet your....)
'atmosphere reversed' is the wordplay.
'atmosphere' becomes 'mood' (I've seen this before).
'reversed' says the letters should be written in reverse.
'mood' backwards is 'DOOM'.
'in' acts as a link.
(Other definitions for doom that I've seen before include "Sentence" , "Downfall" , "Dire fate" , "Inevitable ruin" , "Mood for damnation" .)