The phrase "pernicious influence" refers to a harmful and damaging impact or effect on someone or something. It describes a negative force or influence that can have serious consequences. Full definition
«The new Prevent duty is about protecting people from the poisonous and pernicious influence of extremist ideas that are used to legitimise terrorism. (educationbusinessuk.net)
Take your pick as to who is the most pernicious influence on the club out of the above but it beggars belief that on top of paying a manager 8m a year you need to remind him that coming 4th is not acceptable! (justarsenal.com)
While Pataki, Spitzer, Bruno, came and went, Silver has been the one constant most consistently pernicious influence in state government and is the last man standing. (nystateofpolitics.com)