Armsmaster's program was able to predict the arrival of an Endbringer to Brockton Bay, the gang wars and similar chaos that had occurred making it an ideal target for the Endbringers. The prediction gave the city more warning than most, allowing them to evacuate people and bring in external forces in time.
Armsmaster separated the participants into groups based on their powers and abilities while others handed out armbands. These were designed to alert participants to Leviathan's and teammates' locations, call out casualties and tidal waves as they came, and direct people to the wounded. They suspected his end goal was to use the water in an underground aquifer to sink the city.
The storm and initial tsunami hit the city and the building that the capes were gathered in. Forcing the defenders to teleport outside. Leviathan entered the city and sped towards the heroes once he saw them.
During the first half of the battle Leviathan slaughtered any heroes he got close to by pulverizing them with his sheer mass and after-echo or tearing down the buildings they were standing on. Throughout the battle he dodged most of the ranged attacks thrown at him and simply tanked the rest, including Legend's lasers which made his skin glow from the heat. Leviathan was barely damaged after a direct attack from Alexandria and didn't slow down even after one of Dragon's suits doused him in plasma and self-destructed on him. Eventually, Leviathan did get slightly damaged from the sustained attack, the worst wound being a deep gash across the neck. However every time the heroes even started to get the upper-hand he called in a tidal wave to regain the advantage, killing dozens of participants in the process.
Eventually he got pinned down enough that Miss Militia was able to use some of Bakuda's left over bombs on him. This included one that stuck him to the ground, one that crystallized part of his shoulder, and one that stopped time. Before he could be trapped for good he managed to break free and even threw some heroes into the time-field as he left. He injured Legend and Miss Militia to prevent further attacks before he attacked a group of clustered heroes and villains. Before he could finish them off Clockblocker managed to freeze him and give the heroes time to organize. Once the effect wore off he continued his rampaged.
After killing several villains he got into a one-on-one fight with Armsmaster. Leviathan wasn't able to kill him at first because Armsmaster had a program that predicted his every movement and allowed him to cut chunks of him off with his nano-blade, including his tail. However once Leviathan changed his tactics he was able to defeat Armsmaster and escape. He continued to slaughter heroes and villains alike before disappearing for a time. He later emerged at a damaged shelter and started to slaughter the helpless people inside.
Skitter however distracted him by stabbing him with the nano-blade and drew him back outside. He was further hampered by Bitch's dogs attacking him, but even with him being as damaged as he had ever been he still killed most of them. Finally Scion arrived and began fighting him with the help of Eidolon, forcing a severely injured Leviathan to flee the city.
A memorial to those who had died fighting Leviathan was constructed and placed on Captain's Hill five days after the attack. In accordance with previous battles, it made no distinction between hero and villain, listing a cape identity then a civilian identity if permission was received from family members.