In his Epistle to the Sorrowful Lament upon the Destruction of the Kingdom of Hungary by the Tatars, Roger of Torre Maggiore describes the Mongol Invaders as bloodthirsty killers, who found more joy in killing people than in looting.

During their one year occupation of KoH they destroyed a large part of the kingdom, and reportedly killed everyone who they did not planned to sell as slaves.

Their goal was to destroy, plunder and get hold on valuable resources, gold, silver, cloth, food and slaves, but they also planned to pillage Rome, and other western cities.

The main mongolian army defeated the main hungarian army, but the mongols suffered heavy losses, also the hungarian king managed to flee, so their victory was not full.

After the battle the mongols realized they wont be able to advance much further west. They feared, the population would flee upon they hear about the defeat of their king, and they will go home almost empty handed.

They found the king's seal among the loot, and sent out false letters in the name of the king to the public order: "~Dont fear the wrath of the dogs, dont dare leave your homes, the royal army made a tactical retreat, and with God's help we will soon destroy our enemies."

Some of he fortified castles held out, but the still trusting parishes and smaller settlements became easy prey for the Mongols. They rather chased the fleeing population to death and came back for their values later. You can read about vast destroyed territories, empty settlements in all of the reports on the after invasion Kingdom of Hungary.

After the Mongols left, Bela IV of Hungary's major challenge was to resettle the kingdom and rebuild the infrastructure, thats why he is called the second founder of the state.