Terrifying
Frightening, Dreadful
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of fear? When you really feel fear, you think something is terrifying or frightening or even dreadful.
恐ろしい can describe something that inspires deep fear or awe, often beyond normal feelings of fear. Since it can be used to create a dramatic or intense atmosphere, you might find 恐ろしい more often in literature.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
How terrifying is it? It's Oh so (おそ) terrifying!
恐ろしい夢を見て、真夜中に目が覚めた経験はありませんか？
Have you ever had the experience of waking up in the middle of the night after a terrifying dream?
あんなに恐ろしい目をしたビエトは見たことがありません。
I've never seen Viet with such a terrifying look in his eyes.
我々は、戦争の恐ろしさを忘れてはいけません。
We must never forget the terrors of war.