Final answer:
Charlemagne conquered the Lombards at Pope Leo III's request, declaring himself king of the Lombards in 774, and initiating the formation of an empire by unifying various Germanic peoples.
Step-by-step explanation:
The group Charlemagne conquered at Pope Leo III's request was the Lombards. Charlemagne, king of the Franks, embarked on a military campaign against the Lombards in 773 following the solicitation of Pope Leo III. The Lombards had set up a kingdom in Italy back in 558, having Germanic roots, but they were overpowered by Charlemagne's forces in 774. He took the unprecedented step of claiming the title of King of the Lombards for himself, instead of installing a new Lombard ruler as his vassal. This bold move by Charlemagne was part of his vision to unite different Germanic peoples under his rule, leading towards the formation of an empire that encompassed various territories.