The Skyllian Blitz was a major assault on the human colony of Elysium in 2176 CE. A huge band of pirates, slavers, and batarian warlords, partly funded by batarian financiers, launched an attack into the Skyllian Verge, intending to destroy Elysium. The attack was partly a batarian reprisal for humanity's expansion in the Verge, and partly the result of tensions from the Alliance's pirate suppression campaigns. The decision to finally launch the Blitz was instigated by one man, Elanos Haliat. He intended to use the prestige of the Blitz to cement his position as leader of the Terminus Systems' many pirate bands.
The people of Elysium and the Alliance rallied to defend the colony. Small ground teams made up of Alliance Marines on leave and Elysium's civilians managed to hold off the pirates, while the Alliance Navy, including the SSV Agincourt, engaged their vessels overhead. Navigator Pressly claims that the pirate ships were no match for the Alliance: he couldn't even keep track of how many ships the Agincourt destroyed. The ground teams had a much harder fight, but still managed to hold out for several hours. When reinforcements finally arrived, the enemy turned tail and fled in what vessels they had left.
The aftermath of the Skyllian Blitz led to a major operation on Torfan two years later, to destroy batarian pirate bases. The successful defence of Elysium during the Blitz is often seen as a testament to the Alliance military's training and adaptability. Some batarians remain resentful over the Blitz's failure, claiming it was a just retribution for humanity's actions against them.
If Commander Shepard is a War Hero, Shepard rallied the colonists against the invaders, and when enemy forces broke through the colony's defenses, Shepard single-handedly held them off and sealed the breach. The Commander is rewarded with the Star of Terra and widely regarded throughout the Alliance as a true hero.
During a conversation with Joker, it is mentioned Shepard was present during the Skyllian Blitz, even if they were not awarded any special commendation, regardless of background.