Short answers, pulled from the story.
The title Emperor of the Romans was created on Christmas Day 800 when Pope Leo III crowned Charlemagne in St. Peter's Basilica. This event established the political entity that would later be known as the Holy Roman Empire.
The seven Prince-Electors were the Archbishop of Mainz, the Archbishop of Trier, the Archbishop of Cologne, the King of Bohemia, the Count Palatine of the Rhine, the Duke of Saxony, and the Margrave of Brandenburg. The Golden Bull of 1356 formalized their exclusive right to elect the Emperor.
The House of Habsburg held the title of Holy Roman Emperor for over three centuries starting in the 15th century. They maintained power through strategic marriages, military conquests, and political alliances across Europe.
The Holy Roman Empire ended in 1806 when Francis II abdicated the throne following a devastating defeat by Napoleon at the Battle of Austerlitz. Napoleon had created the Confederation of the Rhine, which forced Francis II to dissolve the Empire to prevent further conflict.
The Investiture Controversy of the 11th century was a bitter conflict between Emperor Henry IV and Pope Gregory VII over the right to appoint bishops and abbots. The dispute escalated to the point where the Pope excommunicated the Emperor, leading to Henry IV traveling to the castle of Canossa to beg for forgiveness.