Bellum Catilinae 48.6

Hello everyone,
I am having some trouble understanding chapter 48 of Bellum Catilinae, namely expression in piece 6
tantam rem esset mentitus
Itaque consulente Cicerone frequens senatus decernit Tarquini indicium falsum videri eumque in vinculis retinendum neque amplius potestatem faciundam, nisi de eo indicaret, cuius consilio tantam rem esset mentitus.
how should I understand tantam rem esset mentitus ? Mentior is usually used with preposition in or de, why accusative?
 

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Belgium
It can be used with the accusative as well. "He had made up something so big".
 
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