Swear by my Sword
(Hamlet, i. 5) - that is, “by the cross on the hilt of my sword.” Again in Winter's Tale, “Swear by this sword thou wilt perform my bidding” (ii. 3). Holinshed says, “Warwick kisses the cross of King Edward's sword, as it were a view to his promise,” and Decker says, “He has sworn to me on the cross of his pure Toledo” (Old Fortunatus).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894