The invasion of Asgard by an alliance between Hel and Loki decimated much of the world's defenses and forced Odin to close off the River of Wisdom from the Yggdrasil as a last resort. After withering for five days, the Nine Realms weakened, paving the way for the apocalyptic Ragnarok.
River of Wisdom Edit
The River of Wisdom was the source of water that gives life to the Yggdrasil. It was closed off however from accessing the chasm below the World Tree by Odin, when Asgard was conquered by Hel and Loki's forces.