I've completed FS, and got the wand, shield and robes of Weyc, but somehow I'm missing the mask. I find it hard to imagine I didn't loot the body of Fyonlecg, so I don't know what happened. Was really hoping to pick up the "One of Them" achieve this run. I'm a bit confused. I guess I need to reload and repeat the Oracle of Wael fight again?

Scratch that. I had the mask, it was the shield (Oracular Focus) that I missed, but I looked up how to get it and now I've completed the achieve.

8 hours ago, Vasvary5050 said:

Scratch that. I had the mask, it was the shield (Oracular Focus) that I missed, but I looked up how to get it and now I've completed the achieve.

Nice

 In case anyone else has the same issue, note there are two possible places to get the mask, depending on the route you took in the forgotten sanctums. 

If you did the stacks before the enclosures (east before west) then you'll find the mask of the weyc on fyonlecg's husk after fighting him, following his "argument" with maura, in the oratory of wael.

If you did the enclosures before the stacks (west before east), then you'll instead fight maura in the oratory of wael, and you won't be able to pick up the mask of the weyc until just before the oracle of wael fight, when fyonlecg finishes his ritual. With some stealth you can grab it off the body before triggering the boss fight. 

Ultimately it doesn't matter which route you take, though doing the collections first is probably easier since you can get the warden contract and make the vithracks friendly, and if you managed not to kill any librarians you won't have to fight the oracle during your first encounter. Also you can still fight maura for her belt (maura's grasping belt) if you revisit the oratory after completing the enclosures as well.