Short answers, pulled from the story.
The Early Middle Ages is the period of European history typically regarded by historians as lasting from the late 5th to the 10th century. It follows the decline of the Western Roman Empire and precedes the High Middle Ages, which ran from roughly the 11th to the 14th centuries.
The term Dark Ages was applied in the 19th century based on the relative scarcity of literary and cultural output surviving from the period. The label is rarely used by academics today, and the alternative term late antiquity is sometimes preferred for the early part of the era to emphasize continuity with the Roman Empire.
The Battle of Adrianople in 378 was a decisive Roman defeat in which Eastern Emperor Valens attacked the Gothic Therving infantry without waiting for reinforcements, only to be overwhelmed when Greuthung cavalry arrived. Only one-third of the Roman army escaped, Valens was killed, and the Roman military writer Ammianus Marcellinus compared it to the disaster at Cannae in 216 BC. The core Eastern Roman army was destroyed, and the Goths were freed to devastate the Balkans.
The Plague of Justinian began in 541 and recurred periodically for 150 years. It is estimated to have killed as many as 100 million people worldwide. Historian Josiah C. Russell suggested a total European population loss of between 50 and 60 percent between 541 and 700. Within Constantinople alone, an estimated 200,000 people, about two in every five city residents, died within less than a year of the plague's outbreak.
Charlemagne was crowned Roman Emperor by Pope Leo III on Christmas Day 800, reviving a title vacant in the west for centuries. He consolidated and expanded the Frankish kingdom into what is commonly called the Carolingian Empire, covering much of modern France, western Germany, and northern Italy. His reign also produced the Carolingian Renaissance, a cultural and educational revival built around the seven liberal arts program designed by the English monk Alcuin of York.
The three-field system, developed in the 9th century, divided farmland so one field grew wheat or rye, a second held a nitrogen-fixing crop, and a third lay fallow. Compared to the earlier Roman two-field system, it put significantly more land under cultivation and allowed two harvests per year, reducing famine risk. It also created a surplus of oats that eventually supported horses as draft animals once the padded horse collar arrived in the 12th century.