Hume on William the Conqueror: Harold Tricked

William of Normandy, not yet William the Conqueror, sees that he may be able to exploit the weakness and disorder of his adversaries to make himself King of England.  His chief obstacle is Harold Godwinson.earl of Essex .  William isn't afraid of using violence when appropriate--he is, after all, the man who won the Battle of Hastings.  But he also knew how to proceed via unction and guile:

By mischance, Harold is delivered up as a prisoner to William:

 William received [Harold] with every demonstration of respect and friendship; and ... he took an opportunity of disclosing to him the great secret of his pretensions to the crown of England, and of the will which Edward [the extant king]  intended to make in his favor. He desired the assistance of Harold in perfecting that design; he made professions of the utmost gratitude in return for so great an obligation; he promised that the present grandeur of Harold's family, which supported itself with difficulty under the jealousy and hatred of Edward, should receive new increase from a successor, who would be so greatly beholden to him for his advancement Harold was surprised at this declaration of the duke; but being sensible that he should never recover his own liberty, much less that of his brother and nephew, if he refused the demand, he feigned a compliance with William, renounced all hopes of the crown for himself, and professed his sincere intention of supporting the will of Edward, and seconding the pretensions of the duke of Normandy. William, to bind him faster to his interests, besides offering him one of his daughters in marriage, required him to take an oath that, he would fulfill his promises; and in order to render the oath more obligatory, he employed an artifice well suited to the ignorance and superstition of the age. He secretly conveyed under the altar, on which Harold agreed to swear, the relics of some of the most revered martyrs; and when Harold had taken the oath, he showed him the relics, and admonished him to observe religiously an engagement which had been ratified by so tremendous a sanction.  The English nobleman was astonished;  ....

David Hume, History of England, Vol. 1, Part A.