Learn German: Nach dem Unfall war die Fahrt die reinste Hölle. - After the accident the drive was sheer hell.

While rein literally means "pure" or "clean", its superlative form is used idiomatically to mean "absolute" or "sheer". You will often see this used for exaggeration in phrases like die reinste Hölle (absolute hell) or der reinste Wahnsinn (pure madness).

English just says "sheer hell." Why does German add the definite article die before reinste?

When using this superlative idiom (der/die/das reinste...) to mean "absolute" or "sheer", German requires the definite article. You cannot drop the article and just say reine Hölle without it sounding unnatural to a native speaker.

Why is die reinste Hölle in the nominative case rather than the accusative?

The verb sein (used here in its past form, war) is a linking verb. It acts like an equals sign connecting the subject (die Fahrt) to its description (die reinste Hölle). Because it is just renaming or describing the subject, the phrase stays in the nominative case rather than taking an accusative object.