When last we saw it, the book of fate was thrown down into the chasm where it remained for ages until about 500 B.C.
(Moore returned to this story at three different points over the course
of Judgment Day, so I've spliced them together as best I can.)
The book was unearthed by trolls serving the wizard Magnar Teufelsun, who used it as his grimoire. Eventually, along came Bram, a Conan-like berserker.
Bram held onto the book through his own rule, but ultimately gave it away to Troll.
It's interesting that even though Troll hinted at doing his own bit of rewriting earlier, it's Merlin who first rewrites large portions of the book of fate to change it, in this case to make Arthur king. It also suggests why Merlin would know so much about the future.
And so the book was locked away and protected by the Knights Templar as the dark ages fell over Europe.
The question that arises is, what was fated originally if Merlin (or more likely Troll before him) hadn't rewritten the book, defeating the wizard Magnar? Was Magnar fated to rule forever? Or had he done his own rewriting to put himself in charge and Troll and Bram were the timeline's way of correcting his changes?
Moore never really answers this, but I think it's a question he meant for us to wonder about.
Next time we'll see how the book first comes to America.