People have asked for the pattern to this so here it is. The original tam pattern is not mine; it was made by Angela Wu and posted here at knitty.com. However since I made a few changes to it I will post the pattern for what I did here. I made a few changes to the charts from what I knitted, to correct some things that didn't turn out as well as I hoped. These changes should get rid of the white bits in the decrease rows, and should make the outside circle more circular.
I've included the Celtic knotwork border pattern as a separate chart since it's not a part of the original design and some people might not want to include it. Keep in mind though that the knotwork pattern repeats 6 times around the whole hat rather than 4.
I'll refer to the charts as A, B, and C in the order they appear.
Gauge:20sts = 4 inches on the larger needles.
Note: Decreases are worked in the following manner: slip 2 together knitwise, knit one, pass the 2 slipped stitches over the knit stitch.
Using a 16-inch size 3 (US) needle cast on 96 sts in main color, place marker, and join in the round being careful not to twist.
Work 7 rounds in 2x2 ribbing.
For the next round, (k4, m1) to end. You should now have 120 stitches.
Switch to a 16-inch size 7 circular needle and begin chart c, repeating 6 times around the hat. If you don't want the Celtic border, knit 18 rows of something else.
Begin charts A and B. Each should appear twice for a total of 4 sections (A,B,A,B). Switch to double-pointed needles when stitches become tight around the circular needle.
When 4 stitches remain, cut yarn and pull through the remaining stitches.
To block, soak tam in water, gently squeeze out, and stretch over a 10.5-inch dinner plate, centering the tam so that the Seal of Rassilon is on one side of the plate and the border pattern is on the other. You may need to hold it in place with one or two rubber bands. Allow it to dry completely.
Although I knit mine with dark yarn corresponding to the dark areas of the seal chart, I think the image might show up better if dark and light were reversed.
The original pattern said to use size 4 for the ribbing, but the hat isn't tight at all even though I used smaller needles for the ribbing. I think that's because the original pattern asked for corrugated ribbing, which is somewhat tighter than normal ribbing.
There are some really long floats in this pattern and can be kind of tricky to keep everything straight. KnittingHelp has some videos on Fair Isle knitting that might be helpful if you need it.