What was the ceremony called in which the vassals promised their loyalty to a lord?

Homage

Explanation:

Homage is the event at which a royal tenant or vassal offered loyalty and obedience to his feudal lord, receiving the sacred title in return for his new position. It was a symbolic acknowledgment to the lord that actually, the vassal was his man. It consisted of the vassal surrendering to the king, symbolized by his kneeling and offering his joined hands to the king, who clasped them into his own, thus acknowledging the surrender. Homage reflects great respect and honor, or something done to honor a person or thing.