I think the codex is a random "drop" from one of those Djinn mobs, not a Lesser Djinn or anything like that. Not a boss either.

I got one codex from a blue archer guy and another from I think a red warrior guy, though the 2nd one had a full group killing so I'm not positive which trash mob it was. It was not a boss though. First drop was from Ao1:3 and it was my first run doing the new quests. The 2nd one probably took like 10 runs all over Ao1, but it dropped in Ao1:2. I also did a few runs in Ao2 during this time; no codex there for me, but that's also a possibility.

The GO TO... kept taking me to Sobek's Blade, so I asked everyone who joined if they ever got a codex and if it was from there. First, most people had not dropped a codex yet, so it could be rarish. 2nd, none of them had it drop in Ao1:4. Don't know where everyone else go their drops, but it sounds like it can be various locations.