Short answers, pulled from the story.
The Night of the Long Knives was a purge carried out in Nazi Germany from the 30th of June to the 2nd of July 1934, in which Adolf Hitler ordered the extrajudicial killing of SA leaders, political rivals, and conservative opponents. At least 85 people died, though estimates range as high as 700 to 1,000. More than 1,000 perceived opponents were arrested.
Hitler acted to eliminate the power of the SA paramilitary force and its chief, Ernst Röhm, who threatened his control by demanding the SA replace the regular army. Hitler also used the purge to settle scores with conservative opponents, Strasserist faction members, and personal enemies, and to reassure the Reichswehr that the SA would not rival the military.
Ernst Röhm was the chief of staff of the SA and one of Hitler's longtime allies who had participated in the 1923 Beer Hall Putsch. On the 1st of July 1934, SS men Theodor Eicke and Michael Lippert entered Röhm's cell at Stadelheim Prison, offered him a pistol loaded with one cartridge, and shot him when he refused to take his own life. Lippert was not tried for the killing until 1957, when he was convicted and sentenced to 18 months in prison.
At least 85 people were killed during the Night of the Long Knives, though some estimates place the final death toll between 700 and 1,000. More than 1,000 perceived opponents were arrested. Victims included SA leaders, two former chancellors, conservative politicians, and at least one accidental victim - Willi Schmid, a Munich music critic killed because his name was confused with an intended target.
On the 3rd of July 1934, Hitler's cabinet passed a measure declaring the killings legal as acts of state self-defence. Reich Justice Minister Franz Gürtner drafted the retroactive statute, which was signed by Hitler, Gürtner, and Interior Minister Wilhelm Frick. Legal scholar Carl Schmitt further defended the killings in an article titled Der Führer schützt das Recht ("The Führer Upholds the Law").
The phrase traces to a ninth-century text, the Historia Brittonum, which recorded the Treachery of the Long Knives, a fifth-century massacre in which the Saxon leader Hengist commanded his men to draw their Seax - long knives - and kill unarmed British leaders. By 1928 the Nazi song "Wetzt die langen Messer" had already used the phrase, and the 2nd of August 1932 issue of The Times noted Nazi leaders invoking "the night of the long knives" publicly. Hitler himself gave the purge that name in his Reichstag speech on the 13th of July 1934.