A downfall is a sudden drop in status or strength. You might be winning at Monopoly until your opponents decide to team up against you, which leads to your downfall.
Poor political decisions can result in the downfall of a government or party, and an economic depression can lead to the downfall of businesses and banks. A more literal, but less common, kind of downfall is when rain or snow falls down to the ground: "The downfall of snow will make the roads slippery tonight." The "decline in strength" meaning has been in use since the 14th century.