Short answers, pulled from the story.
David Rizzio was an Italian courtier and private secretary to Mary, Queen of Scots, murdered in Edinburgh on the 9th of March 1566. The killing was led by Patrick Ruthven, 3rd Lord Ruthven, and carried out with the involvement of Mary's husband Lord Darnley. Darnley's motivation was jealousy over Rizzio's close relationship with the Queen and his growing political influence.
Rizzio arrived in Scotland in 1561 as part of the retinue of Carlo Ubertino Solaro, Count of Moretta, on a diplomatic embassy. When the Count dismissed him, Rizzio was taken on as a musician-valet in Mary's household, using his bass voice to complete her chamber ensemble. Towards the end of 1564, he was appointed her secretary for relations with France, replacing Augustine Raullet.
David Rizzio was murdered in the supper room and adjacent audience chamber of the Palace of Holyroodhouse in Edinburgh, on the 9th of March 1566. He was initially buried in the Holyrood Abbey cemetery, but Mary later had his body moved to the tomb of her father James V and Madeleine of France within the abbey. The location of the murder is still marked with a plaque and a red mark on the floorboards in the Audience Chamber.
The conspiracy was led by Patrick Ruthven, 3rd Lord Ruthven, and included Lord Darnley, Morton, Lord Lindsay, William Maitland of Lethington, and George Douglas. Thomas Randolph documented a list of more than two dozen participants and suspected accomplices. In the weeks after the murder, many of those named had fled toward Newcastle in England.
Mary, Queen of Scots escaped from Holyroodhouse at midnight the night after the murder, riding to Seton Palace and then to Dunbar Castle. She returned to Edinburgh and on the 21st of March had Lord Darnley publicly declared innocent of the murder. The killing contributed to the downfall of Darnley and had serious consequences for Mary's subsequent reign.
John Carradine played Rizzio in the 1936 RKO film Mary of Scotland and Ian Holm in the 1971 Mary, Queen of Scots. Ismael Cruz Córdova took the role in the 2018 film Mary Queen of Scots, and Denise Mina published a novella titled "Rizzio" in 2021. Arthur Conan Doyle used his death in his 1908 story "The Silver Mirror," and the 1830 play Maria Stuart by Juliusz Słowacki made the killing its central subject.