cast > [he] resolved despite > {Outrage, act of defiance}
4 Which he had borne of his bold enemy.
of > from
5 Tho to his brethren came: for they were three
Tho > Then brethren > brothers
6 Ungracious children of one graceless sire,
Ungracious > {Lacking in heavenly grace; ungracious} sire > father
7 And to them complained how he 8 Had used been of that foolhardy squire;
of > by
9 So them with bitter words he stirred to bloody ire.