Since the dawn of time, the Kingdoms of Heaven and Hell have been warring against each other. During these wars, the Charred Council came into existence to keep balance and ensure order. Their enforcers, the Four Horsemen, have stopped or killed anyone who broke the laws of the Charred Council.
Eventually, the Kingdom of Man came into existence. The Charred Council noticed their relative weakness to those of Angels and Demons but also noticed their extreme cunning and guile. They foresaw the importance of this Third Kingdom in maintaining this balance and dictated that the armies of Heaven and Hell would rest until Seven Seals were broken, marking the beginning of Armageddon during which the Kingdoms of Heaven, Hell, and Men would fight their last war.
You are the second Horseman, War and the Seven Seals have been broken. You have been summoned to Earth for the beginning of the Apocalypse.
For those of you seeking to get all the achievements as quickly as possible, all achievements CAN be obtained in a single playthrough by playing on the Apocalypse difficulty. However, it may be beneficial to play through the game on a lower difficulty because over the course of your first playthrough, you can construct the Abyssal Armor which will then be available at the start of the game on all subsequent playthroughs. While it may not sound like THAT big of a deal, it really is. Not only does the Abyssal Armor reduce the damage you take be a significant amount (I'm unsure of the amount but possibly as much as only 10-20% damage) but it also automatically gives all your weapons a weapon leeching ability. Even on Apocalypse difficulty, it essentially allows you to play as if the game were on easy. If you feel confident in your abilities, feel free to play through on Apocalypse difficulty because it's still not particularly hard but if you are unsure or new to the genre then know that you can make it infinitely easier on yourself. Just make sure that you choose Abyssal Armor at the beginning of your second playthrough because you won't be able to equip the armor later on if you decide to use it.
Also, for those concerned with missable achievements, there are three achievements that can be outright missed and a fourth which becomes much harder to get the further into the game you go. These achievements are "Open Air Parking" during the prologue where you have to hit a helicopter with a thrown car, "Aerial Predator" which requires you to get 160 kills during an on rails section and "Who's Counting" which requires you to get more kills than someone during a particular section. You can save at any point during the game and the walkthrough will include all of these achievements. Try to save before so you can retry.
The fourth achievement is not "missable" per se. It is a cummulative achievement called "Improvised Kills" which requires you to get 150 kills using items in the environment. This basically means that you have to kill enemies with thrown items like cars and thrown bombs. It may also include environmental explosions but I'm not sure. The reason why this gets difficult later on is that no breakable environmental items respawn at any point during the game so if you accidentally break something even if it's just by walking into it, it will not respawn. What this means is that later, you'll have very few items to use, particularly in the earlier areas where the weakest enemies are (which is the easiest spot to get kills). You may even be forced to go to an area with growing bombs and use those but they are frequently in puzzle areas, not combat areas making them difficult to use for this purpose.