A witch’s/an evil brew
Definition 1 of 3
Pro Tip 1/2Negative Connotations
The phrase often implies something intentionally harmful and sneaky. 
That plot to take over the company was a witch's brew of corporate espionage.
Pro Tip 2/2Cautious Usage
Avoid using in sensitive contexts as it suggests deliberate malice or evil. 
The politician's campaign strategy was described as a witch's brew.