Recently I picked it up in Phomunia Aquadacts while doing CoP Mission with LS mates, so I figured I'd give this NM a go.
http://ffxi.allakhazam.com/images/items/FCodex.JPG
Drops off Stegatours.
The conditions to popping this NM in Sacrarium requires extreme amount of Fomor hate. Meaning that they will aggro you even if you have sneak on when you try to get to "???". (Note on fomor hate: They won't aggro you at all if you have 0 hate and unsneaked if I'm not mistaken.)
The codex itself is used to pop Fomor NMs at 5 different spawn points. Each spawn point pops a Fomor of different job class. There's a THF(Elvaan), a BRD(Hume), a RNG(Hume), a RDM(Hume) and a BLM/SMN(Taru). I was told that you have to pop it a point corresponding you race, but seeing as 3 out 5 are Humes I don't see how that can be the case.
The item you get from killing the NM, aside from RSE throwing Rare/EX items (which seems to have been removed from somepage's list) is this:
Swift Belt
So my question is to everyone out there who has done this fight. What's the difficulty of the NM? How hard is it to get to pop point with such extreme amount of hate?