 The major source for Tolstoy as he was working upon the drama (as it was with the whole trilogy) was "History of the Russian State" by Nikolay Karamzin.The whole of the Act 1 is based upon one small fragment in Vol.IX of it describing Ioann's feelings after the killing of his son, his relinquishing the throne, boyars' reaction to this and his consent to (as he put it) "bear this burden of rule for some more time"  Details of this fragment of "History" have been expanded by Tolstoy into full-blown scenes