Extremely Gruesome
Extremely Fierce, Extremely Violent
noun, な adjective
Something really terrible causing mass extinction will be extremely gruesome. It will also probably be extremely violent, so don't look.
せいぜつ
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!
殺人現場は凄絶だった。
The murder scene was extremely gruesome.
私の祖母は戦争を経験し、凄絶な時代を生きました。
My grandmother experienced a war and lived in an extremely fierce era.
たった今、世界の凄絶さを目にした気がした。
I feel like I just saw the extreme violence of this world with my own eyes.