I don’t remember the details of their particular agreement, but traditionally a vassal and a liege swore mutual protection - the vassal would provide men, and the liege would use those men to protect the lands. This oath was sworn on their honor. Honor was such a huge part of feudal society that the last duel in the USA was fought in 1859. Breaking that oath in this case literally meant that they forfeited their souls.
I don’t remember the details of their particular agreement, but traditionally a vassal and a liege swore mutual protection - the vassal would provide men, and the liege would use those men to protect the lands. This oath was sworn on their honor. Honor was such a huge part of feudal society that the last duel in the USA was fought in 1859. Breaking that oath in this case literally meant that they forfeited their souls.