You play as War, the first of the Four Horsemen of the Apocalypse. Long ago Heaven and Hell warred endlessly, until the Charred Council arrived to broker a ceasefire as the kingdom of man emerged to protect them until they were ready for the end war. When all three kingdoms were deemed ready for war, the Seven Seals would be broken, signaling the end of the truce and beginning of the end. Destroying the seals also has another effect, calling the Four Horsemen, enforcers of the Charred Council and feared by demon and angel alike.
But, before the kingdom of man was ready, someone broke the seals early, all but one. This triggered a false call that War answered, descending upon Earth even as war between Heaven and Hell began anew with humanity caught in the crossfire. When the Charred Council realized Heaven and Hell were once more fighting and that War was as well, they blamed the early war and humanity's destruction on him and sentenced the Horseman to death. However, having a strong sense of honor and justice, War proposed another answer, letting him find the true culprit. If he succeeds, his name is cleared and the real villain will be punished, if he fails, he will die anyway and the judgement of the Council will be done. The Council agrees, but places The Watcher within War to bind him, restricting his true power and being able to inflict pain to ensure War does his job.
Fast forwarding a bit, the true culprit is discovered, an Angel who led the armies of Heaven, Abaddon. He despised Hell and believed his forces could subdue those of Hell, but was too impatient to wait for the end war. So he kick started it, by contracting Ulthane (a legendary blacksmith and a Maker, who are a race that are older than the kingdoms of heaven, hell, and man.) to help him break 6 of the Seven Seals, triggering the false call to the end war and having Ulthane reforge the seals to cover up his deeds and make it seem as if it was the demons who broke the truce. What Abaddon didn't count on was War showing up, he protected the final seal specifically to prevent the arrival of the Horsemen. War's arrival in the beginning of the early war distracted Abaddon, leading to his apparent death.
After his "death", Abaddon was approached by the Demon Queen Lillith, who made him an offer, "Serve in Heaven, or Rule in Hell?". Broken down by the failure of his plan, Abaddon took her offer and became the Destroyer, leading Hell's armies on Earth. After aiding the Angel of Death Azrael, the angel brought War to the hidden land of Eden, where War received a vision of the future from the Tree of Knowledge, a vision he only partly revealed to The Watcher. Upon returning to Earth, War set about reforging the Armageddon Blade, a powerful sword to help him kill Abaddon. In the process, the new leader of the Angels still on Earth, Uriel, challenged War, still believing he was a criminal. She challenged him to Nex Sacramentum, a Death Oath, they would fight until one or both died. War accepted and easily beat her, but did not kill her. Instead he told her what happened to Abaddon, knowing she would lead the Angels against the Destroyer.
After killing Abaddon who dropped the Seventh Seal he had kept to guard, The Watcher exerts his power over War, revealing that the Charred Council knew War was innocent and that he would kill everyone truly responsible for them to clear his name. Due to The Watcher's sadistic nature, he gloated and in doing so gave Uriel an opening. She impaled War with her sword, killing him and fulfilling the Death Oath. Surprised, The Watcher left himself open as Uriel broke the Seventh Seal. With the final seal broken, War was not only revived to answer the call, but now in possession of his full unrestricted power. The Watcher had no more power over him. War easily killed The Watcher. Uriel realized the truth, that War had known what would happen. War stated the true reason he spared her before was that "I would not see the last of Heaven's honor die with its champion."
Uriel declared all debts repaid to War, and allowed him to leave (though she couldn't have stopped him anyway). Before the Horseman could exit, she questioned him, Heaven, Hell, the Charred Council, and others will hunt him, she asked if he would wage this war alone. In answer, War held up the shattered remains of the final seal, simply stating "No. Not alone." as his fellow Horsemen descend in the distance.