Malevolent Possession

What are the symptoms of nonconsensual possession by a malevolent spirit? I don’t want to elaborate too much for fear of skewing the answers, but I have reason to believe that someone I know very well has been under malevolent influence for a very long time. I want to explore the concept further.

Honestly, no such thing as an non consensual possession it’s more like unwilling to fully commit to it.