The black death started in by the fleas on rats which spread diseases to people and places by trade.
The fleas then went to humans and spit the bad blood in their bloodstream because of cuts. Humans would get symptoms like Bubus and their skin turned into ashes and their bodies decayed. to stop the infection many people try to get leeches to suck out the bad blood from their body and beat each other attempting to beat the infection from their bodies or they thought that the plague was a sin and that beating themselves would make God happy.
This will make god forgive me...
BRING OUT YOUR DEAD!
unfortunately, these attempts to stop the disease were unsuccessful and resulted in death. This declined feudalism because so many people would have died so there would be fewer people to do the work. In this case, the surviving peasants demanded higher pay and more to do the work.
The 100 year war
The 100 year war was a battle between France and England that started because the king died without having a written heir to the Throne.
The throne should be my sons because he is his niece!
The throne should be mine because I am his cousin!
Because of the war monarchs on both sides had collected taxes and had raised large armies.
The 100 year war declined feudalism because the big armies they had made the Nobels job more unimportant to the monarchs because they no longer needed The nobles to supply knights for them.