The Siege of the Chaos Cannon was a battle that occured on Apokolips between the Batman and Darkseid. It was a battle to try and resurrect Damian Wayne, but to also stop Kalibak from using the Chaos Cannon on a planet with life. A conflict that affected the Multiverse, the Siege finally let the conflict between Batman and Darkseid come full circle.
The machinations of the Siege started when Darkseid started to track Kaiyo the Trickster, forcing her to try and use the Chaos Crystal of Earth 2. When this failed, the crystal split into shards across the multiverse, a Crystal Shard landing in Earth 0, to be picked up by Talia al Ghul. al Ghul used it to enhance and rapidly age her and the son of the Batman, Damian Wayne. Darkseid then launched the Parademon Invasion of Earth. The Justice League of Earth 0 managed to repel him, but ended up stuck on Apokolips in a later unrelated mission. Darkseid, during this time, was trying to heal from the wounds the Justice League inflicted on him and took note that the Batman was left behind. Having his minions try and clone an army of Batmen, Darkseid was angered upon their failure and from a distance, used the Omega Sanction to send Batman back in time. Trying to heal, he let his son Kalibak take control of Apokolips and the lesser God, with the aid of Glorious Godfrey, designed the Chaos Cannon. Eventually locating al Ghul's shard on Earth 0 and another piece embedded in the sarcophagus of Damian Wayne, Godfrey recovered both in the Battle of Nishapur. Enraging the Batman, he recovered his Hellbat suit and used the Boom Tube system to get himself to Apokolips. Jason Todd, Barbara Gordon, Tim Drake and Cyborg later followed him there as well.
On Apokolips, Batman began attacking the Parademon Army and the Justifiers in his search for Glorious Godfrey. While doing this, Kalibak used the Chaos Cannon to destroy one of Apokolips' moons. Finding Godfrey, Batman brutally injured while learning of Kalibak's malicious intent to use the Cannon to destroy a planet with life. As the rest of the heroes arrived, fighting Hellhounds, they soon met up with Batman. He then took out a bulk of Godfrey's Parademon Army to make his way to Kalibak's compound. By this time he had garnered the attention of Darkseid, who left the shadows of healing to confront the dark knight. In fighting Kalibak, they also destroyed the Chaos Cannon just as Darkseid arrived. Batman fought Darkseid alone and although easily defeated, managed to stall long enough to use the Chaos Shard to absorb the Omega Sanction and Boom Tube his family and allies back to Earth 0.
Returning to the Batcave, Batman used the Chaos Shard, powered by the Omega Sanction, to resurrect his son. The implications the battle had for the Multiverse, however, was far greater as Darkseid realized he was not healing fast enough. Tying in right to the heart of the Second Apokolips War, Darkseid used the heart of Apokolips to heal himself. This began to kill his planet, forcing him to try and replace it with Earth 2.