The murder scene was extremely gruesome.
My grandmother experienced a war and lived in an extremely fierce era.
I feel like I just saw the extreme violence of this world with my own eyes.
Something really terrible causing mass extinction will be extremely gruesome. It will also probably be extremely violent, so don't look.
This is a jukugo word that uses the on'yomi readings of the kanji. You've learned them already, so you should be able to read this on your own!