。
The confusion was truly beyond description.
Literal
That confusion truly nameable-state-[must-not-すべからず].
名状すべからず is a classical construction: 名状 'to name / describe (a state)' + すべからず (classical negative imperative/prohibition of する 'must not / shall not'). The literal force is 'it must not be named,' idiomatic for 'indescribable / defying description.' 実に 'truly / indeed' is a formal literary adverb. The whole sentence is in a stiff, almost archaic written register — the kind of phrasing found in old newspaper writing or literary prose.