The idiom "the die is cast" is well known. The simple past tense version of this in the indicative mood should be, "the die was cast." In the novel, The Godless, by Ben Peek, I came across the sentence, "He knew there was something wrong with this, a gut-level reaction, but he knew that the die were cast now." Unless I am missing something, this sentence is in the indicative mood.
I then did an online search and found several books that used "the die were cast" in the indicative mood, so this doesn't appear to be a typo. Examples included Colombia and the United States: Hegemony and Interdependence, by Stephen J. Randall; Perspectives on the Nuremberg Trial by Guénaël Mettraux; A Stubble-Jumper in Striped Pants: Memoirs of a Prairie Diplomat, by Earl G. Drake; Writing History in Renaissance Italy, by Gary Ianziti; etc. I understand "the die were cast" is correct if it’s being used in the subjunctive mood in a clause like, "if the die were cast," or, “as though the die were cast,” but that is not the case in any of the cited examples.
My question: Is the use of "the die were cast" in the indicative mood an idiom, or a regionalism? If it is, does anyone know its etymology in the specific form of "the die were cast" when being used in the indicative mood (I am interested in knowing how a singular noun came to be the subject of a plural verb)?