Learn how to say Perniciousness correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word pernicious:
having a harmful effect, especially in a gradual or subtle way:
the pernicious influences of the mass media
late Middle English: from Latin perniciosus ‘destructive’, from pernicies ‘ruin’, based on nex, nec- ‘death’
The ending of pernicious is spelled -cious.