The beginning of the end
Definition 1 of 1
Pro Tip 1/3Negative Outcome
Often suggests something will end badly rather than just concluding. 
When the star player got injured, fans feared it was the beginning of the end for the team's winning streak.
Pro Tip 2/3Inevitable Conclusion
Implies that the end result is unavoidable once this point is reached. 
The merger talks stalling was the beginning of the end for the small company's independence.
Pro Tip 3/3Dramatic Change
Marks a significant or dramatic turning point, not a gradual decline. 
The sudden drop in product quality felt like the beginning of the end for the brand's reputation.