Learn Arabic (Modern Standard): الظلم شيء خطير.

In Arabic, abstract concepts and general ideas require the definite article ال. While we just say Injustice in English, Arabic uses the definite article to talk about the concept as a whole.

Is الظلم a verbal noun (masdar)?

Yes, it is the verbal noun for the basic Form I verb ظلم (to oppress or do wrong). It follows a very common Form I pattern where the middle root letter has no vowel.

How do we know خطير describes شيء and not الظلم?

Adjectives in Arabic directly follow the noun they modify and must match them in definiteness. Since خطير (dangerous) and شيء (thing) are both indefinite (they do not have ال), they pair together perfectly to mean a dangerous thing.

Where is the word for is in this sentence?

This is a nominal sentence, which consists of a subject and a predicate without a verb. In Arabic, present-tense sentences like this do not use a word for is; it is simply understood from the two parts of the sentence being placed next to each other.