Googled up and down the internets for a walkthrough. Found nothing. So I pretty much done everything in there. Turned every stone upside down. Can't find a way to progress. I got in there. Got beaten by the scaleless dragon. Freed myself. Freed Big Hat Logan. Got the ember and all the other loot. I can come and go as I please, but I don't know how to progress to this area's boss. All I need is this last boss soul to fill the Lord Vessel. How and where in Duke's Archives do I progress after freeing Big Hat Logan?

Where Big Hat is after you free him, in the other room there's a switch you can pull to reveal some stairs.

In the room where you found the key to logan's cell, there is a switch that opens a stairway leading down. Take that and head outside to the crystal caves. Seath is hiding in there.

@Ezekeilpurger said:

Where Big Hat is after you free him, in the other room there's a switch you can pull to reveal some stairs.

Hmm? He just disappeared. Didn't he return to Firelink? What part of the Archives is he at now?

@Chop said:

In the room where you found the key to logan's cell, there is a switch that opens a stairway leading down. Take that and head outside to the crystal caves. Seath is hiding in there.

Thx man. Missed that lever.

@Seppli:

No he stays in the library in the archives and sells sorceries to you.

Funny thing.
I rescued him but he never appeared in that final room.
Tried quitting and loading and resting after rescuing him, nada
All I wanted was to kill him when he became a phantom :(