A Staten Island lawyer — who

challenged another attorney to a “Game of Thrones”-style trial-by-combat duel

— pleaded guilty to extortion and fraud on Monday in Brooklyn Federal Court.

Richard Luthmann, 39, coped the plea deal in connection with a bogus scrap metal business that took advantage of a disabled man to conceal their fraud and collected debts at gunpoint in Luthmann’s law office.

As part of his plea deal Luthmann also stipulated that he sent a threatening letter to a potential witness he believed was cooperating with the feds.

A judge ordered Luthmann to undergo a psychiatric evaluation in June 2018 after he sent the letter.

Luthmann faces up to 6 1/2 years in jail for the two counts of wire fraud and extortion. He will be sentenced at a later date.