The siege of Storm's End was an important part of the War of the Usurper. Robert Baratheon's army had been driven from the south by the forces of Mace Tyrell, leaving the stormlands largely unprotected. Robert named his brother Stannis the commander of the garrison he left to hold his ancestral castle.
Stannis was young and untested in battle, and most expected him to surrender when the Tyrell army laid siege to the castle. Paxter Redwyne's fleet had closed Shipbreaker Bay to any ships that might try to supply the beleaguered defenders. Stannis proved stubborn, however, and he held Storm's End through a full year of siege, despite the fact that his own men were starving. They were nearly forced to eat their own dead.
Fortunately, before they were forced to that extreme, a notorious smuggler named Davos braved the stormy bay and Redwyne ships to land a small boat under the castle. The food he brought allowed the garrison to survive long enough for Eddard Stark to lead a rebel army to their defense. There proved to be no battle; the Battle of the Trident already been fought, and as soon as the rebels appeared on the field, Mace Tyrell surrendered.
Davos was given a knighthood for assisting the garrison, but Stannis decreed that he must lose one joint from each finger on his right hand for his years of smuggling. Forthwith, Davos was known as Shorthand, and took the name Seaworth for his new House.