The verb subside means a condition that becomes less strong or extreme.

Synonyms are die down, diminish, ease, or abate.

The word origins from Latin (1680s) subsidere meaning “sit down, settle, sink, fall, remain, crouch down, squat”. It comes from sub meaning “under, beneath” plus sidere meaning “to settle”, related to sedere meaning “to sit”. It was used of liquid surfaces, “to sink to a lower level, be reduced” and is from 1706.

The water began to subside this morning. 

His fever subsided because of the natural medicines from his grandma.

My father’s anger subsided as he listened to my mother’s calm voice.