The Mongol Empire was founded in 1206 by Genghis Khan, whose birth name was Temujin, which is Mongolian for "iron," when he went on a war path to conquer most of Eurasia. After Genghis Khan's death in 1227, his sons took up the cause. His youngest son Ogodie, particularly took up the cause in 1236 to conquer Europe. By 1240, the Mongol Empire included Russia, the Ukraine, Romania, Bulgaria, and Hungary.
The Mongols were famous and feared for their warfare style of annihilating entire dissenting villages, which had a major effect on European population. Entire European populations were massacred, and survivors fled as refugees, severely changing population demographics. In addition, the Mongols are believed to have been responsible for the bubonic plague that annihilated over a third of Europe's remaining population. The bubonic plague is carried by fleas of marmots, large squirrels that live in Central Asia's steppes, and these fleas were carried over by the Mongols, leading to the Black Death.
However, after the invasions, under Mongolian rule, Europe fell into a period of 100 years of peace, called by historians the "Pax Mongolica." During this period of peace and trade between Europe and China along the Silk Road increased, leading to prosperity as well as cultural development as multiple cultures shared goods, spices, religions, and philosophies.
By the 1300s, the Mongol Empire was the largest in the world, with land from China to Eastern Europe. When Genghis Khan died in 1227, his descendants started this empirical expansion into the Middle East, and soon into Europe. This caused many refugees to flee towards Western Europe, as Mongols often depopulated entire towns that they went through. It is also likely that when the Mongols invaded Europe, they brought a deadly disease with them from fleas, called the bubonic or black plague. From history, 1/3 of Europeans died from this disease. A positive outcome that came through was the increase in cultural exchange and trade between China and Europe along the reopened Silk Road. Inventions such as guns and gunpowder made its way to the west. This had an enormous impact on Europe's economy, as merchants carried new inventions into Europe.