I have 5 of the 6 badges and I'm assuming Seth must have 1. I saw him when entering the castle and he disappeared. I've now pretty much gone through the castle and can't find anything else to do. Can anyone tell me where I find Seth?

Seth will appear in one of the rooms in Marketh's place after you've finished talking to Marketh.

But Seth doesn't have a badge. So the badge you're missing is someone else's. Which badges do you already have?

The six badges are:

1. Found on Krilag, the orog in chapter 4.
2. Found on Joril, the frost giant jarl in chapter 5.
Chapter 6:
3. Received from Marketh in his palace.
4. Found on Malavon in HIS palace.
5. Received from the high priest in the forsaken temple.
6. Found on Maiden Ilmadia in the great forge.

Hope this helps.

#1 and #2 are the easiest to miss. If you've missed one of these, #1 is in a dead-end cave off of the drow area in Upper Dorn's, and #2 is across a rope bridge you have to repair in Wyrm's Tooth.