次の3点をクリアにしたいので、相談させてください

1 the end of it all の部分で、なぜ単数のitなのか
2 allの意味はどういう意味なのか
3 文脈に合わせた訳も教えてください

よろしくお願いします

前文
Thor was just as strong as the Mighty Thor in the comics, his hammer as powerful, but he was . . . well, honestly, not the brightest of the gods; and Loki was not evil, although he was certainly not a force for good. Loki was . . . complicated.

質問の文
In addition, I learned, the Norse gods came with their own doomsday: Ragnarok, the twilight of the gods, 【the end of it all.】 The gods were going to battle the frost giants, and they were all going to die.