The term 'Vassals Oath' refers to a pledge of loyalty and service that was made by a vassal to their lord in the Middle Ages. The crossword solution for this term is 'FEALTY, ' which is defined as a strong sense of loyalty and devotion to a person or cause. This term was used to describe the relationship between a vassal and their lord, in which the vassal would swear an oath of FEALTY to their lord in exchange for protection and land.