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What the lawyer told me turned out to be a lie in the end.
Literal
Lawyer [subject-が] me [to-に] said [nominalizer-こと] [topic-は] finally lie [that-であること] [subject-が] became-known.
Two nested nominalizations. (1) 弁護士が私に言ったこと is a relative-clause-like nominalization: 'the thing the lawyer told me.' (2) 嘘であること is a copular nominalization: 'the fact that it is a lie.' The matrix verb わかる ('become known, come to be understood') takes the second nominalization as its subject. とうとう ('finally, in the end') signals the eventual resolution of uncertainty.