Patch 4.3 signifies the end of the story of Cataclysm, the moment of Deathwing's demise. Much like patch 3.3 before it, 4.3 features three dungeons that are directly related to the endgame content -- what you do in the dungeons is 100% part of the story that leads to the Dragon Soul raid. So we have End Time, the Well of Eternity, and the Hour of Twilight, all three offering different parts to this story.
All of this information has led to hundreds of questions. If we kill Murozond, what happens to that alternate future? If we take the Dragon Soul from the past, doesn't that mean our future is irrevocably altered? Perhaps most important is the last cinematic for the Dragon Soul fight, which was leaked earlier this week and raises way more questions than it answers.
Please note: Today's Know Your Lore is full of spoilers for patch 4.3 -- we're talking everything from the story behind the new heroics to the new short story to the end of the Dragon Soul raid. It also holds spoilers for the novel Thrall: Twilight of the Aspects. If you're avoiding spoiler content, turn away now.