This achievement is supposed to unlock after collecting all 20 pages of the warlocks journal, the pages drop from monsters randomly upon death. However I have yet to hear one case where someone actually had to find all 20.
I unlocked this while still missing three pages #'s 9,13,18, since then I have found page 13 but the other two will not drop. This does not seem to be a problem since there are no cases of the achievement not unlocking, just unlocking prematurely.
Feel free to post updates and experiences so that the guide may be more complete.