Short answers, pulled from the story.
Sigismund of Luxembourg (the 15th of February 1368 - the 9th of December 1437) was Holy Roman Emperor from 1433 until his death. He also held the titles of King of Hungary and Croatia, King of Germany (King of the Romans), King of Bohemia, and King of Italy, making him the nominal temporal head of Christendom.
The Battle of Nicopolis, fought between the 25th and the 28th of September 1396, was a crusade led by Sigismund against the Ottoman Empire. Sultan Bayezid I arrived with ten thousand men and completely defeated the Christian force of fifteen thousand under Sigismund, who escaped by sea. The defeat destabilised Sigismund's rule in Hungary.
Sigismund founded the Order of the Dragon after his victory at the Battle of Dobor in 1408. Its main purpose was fighting the Ottoman Empire. Founding members included Stefan Lazarevic, Nicholas II Garay, Hermann II Count of Celje, Stibor of Stiboricz, and Pippo Spano, along with the most important European monarchs of the era.
Sigismund was one of the driving forces behind the Council of Constance (1414-1418), which ended the Western Schism by resolving the dispute between three rival claimants to the papacy. He travelled to France, England, and Burgundy during the council's sittings to press for the abdication of all three rival popes, and used the weakness of Antipope John XXIII to secure the council's convening.
After the death of Wenceslaus IV in 1419, Sigismund was the nominal King of Bohemia but waited seventeen years for the Czech Estates to acknowledge him. The Bohemians blamed him for the burning of Czech reformer Jan Hus at Constance in July 1415. Three military campaigns against the Hussite rebels ended in defeat, including losses at the Battle of Kutna Hora and the Battle of Nemecky Brod at the hands of Jan Zizka.
Sigismund died on the 9th of December 1437 at Znojmo in Moravia. As he had ordered during his lifetime, he was buried at Nagyvarad in Hungary (today Oradea in Romania), next to the tomb of King Saint Ladislaus I of Hungary, whom Sigismund deeply venerated.