IPA: /ˌpɛstəˈlɛnʃəli/
KK: /pɛstəˈlɛnʃəli/
In a manner that causes or relates to disease or plague; harmful or destructive.
The air was pestilentially filled with the stench of decay.
Pestilentially → It is formed from "pestilent" (from Latin "pestilens", meaning causing plague) and the suffix "-ially" (meaning in a manner). The word "pestilentially" means in a manner that is harmful or causing disease, akin to a plague.
Think of something that is 'causing plague' ('pestilent') and how it can be done 'in a manner' ('-ially'). This helps you remember that 'pestilentially' refers to something harmful or disease-causing.