In Sack and Ashes - Emperor Henry IV's Walk to Canossa

25 January 1077, the banned King of the Romans, Henry IV, arrived at Castle Canossa in Northern Italy to do penance and ask pope Gregory VII for forgiveness. 

“In the courtyard at Canossa / Stands the German Emperor Henry, / Barefoot in his shirt of penance, / And the night is cold and rainy. // From the window two dim figures / Gaze upon him, and the moonlight / Gleams on Gregory, bald-headed, / And the white breast of Matilda. // And the pale-lipped Emperor Henry / Prays his pious paternoster; / But within his kingly heart he / Rends himself and cries in anguish : // " Far in my own German country High and mighty hills are towering; / And within their depths, the iron / For the battle-axe is growing. // " Far in my own German country / High and mighty oaks are towering; / And in some great trunk, the handle / For the battle-axe is growing. // " Germany, my own beloved, / You will bear the mighty hero / Who will wield the axe and swiftly / Crush the serpent that torments me!" (Heinrich Heine, “HENRY Auf dem Schlosshof zu Canossa” 1844)

Eduard Schwoiser’s (1826 — 1902) painting
“Heinrich vor Canossa” (1862),
depicting the king in the same spirit of defiance as Heine did in his poem

The Walk to Canossa, Gang nach Canossa. It was a climax in the Investiturstreit (Investiture Controversy), i.e., who had the right to invest bishops and abbots in the Holy Roman Empire, the king or emperor or the pope. Since bishoprics and abbeys where usually connected with huge fiefs and political power, this was an essential question about who had the say in the empire. And with rebellious princes at his back at home in Germany, the banned Salian king had no choice but to comply and abase himself before the pope. 
Tradition has it, that Henry wore a penitent hair-shirt and walked barefoot after crossing the Maritime Alps by the Mont Cenis pass, the southern German dukes had barred the king’s way to Italy via Switzerland, and when he arrived at Castle Canossa in the Reggio Emilia, 20 miles south of Parma, the pope had the king waiting for three days in his penitential garb at the closed castle gates until he condescended to admit the humbled monarch. 

August von Heyden (1827 - 1897): "Henry IV at the Gates of Canossa"

The ban was lifted and Henry had the necessary leeway to straighten out the situation in the Holy Roman Empire over the next three years. Henry was banned again in 1080 but with most of the German princes and the clergy on his side, the King of the Romans returned to Italy with an army, laid siege to Rome and finally deposed Pope Gregory in 1084.The Walk to Canossa, as well as "in Sack und Asche gehen" (more or less - walk in dust and ashes) became winged words in Northern Protestant nations, especially Prussia and the empire of 1871 for doing something humiliating, a tradition that began during the “Kulturkampf” (lit “culture struggle”) about who had the interpretational sovereignty in the state, government or the Catholic Church.