Anglo-Saxon men actually wore tunics much shorter than Slytherin's, but as his statue in the Chamber of Secrets has floor-length robes, I did my best to stick with canon. The solution is that, like the contemporary Ottonian emperors, Slytherin emulated the longer tunic style of the Byzantine upper class. The Byzantines were seen by some as the heirs of the Roman Empire and the height of medieval civilisation. Similarly, the length of Slytherin's beard is also a symbol of his status; for English (and Germanic in general) men, the length of beard and hair was very important in determining social rank in the early Middle Ages.
Slytherin is showing off Europe's finest needlework, that of Anglo-Saxon seamstresses, in his cloak and tunic. So much gold was embroidered into the clothing of wealthy nobles that their garments were often later burned for the hundreds of pounds worth of gold within them. There are few surviving examples of medieval English embroidery, so the designs here are also inspired by medieval English metalwork, leatherwork, and illuminated manuscripts. His sleeves and hose are wrinkled in the English style, and his cruciform brooch is of an early medieval East Anglian design. Black clothes were uncommon but were notably popular among the Danes, who controlled Slytherin's native East Anglia during much of his lifetime. The dye was achieved by adding iron to other dyes. Dark green was a mixture of weld (yellow) and woad (blue).
The main issue was his locket. The design in the books and movies is quite Victorian. It's such an important artefact that I couldn't do away with it, though. The locket pictured is a medieval locket (although the design may postdate Slytherin by a few hundred years). They were not worn around the neck but were used for storing saints' relics, medicine, talismans or poison. Perhaps through the centuries as the locket was damaged and fashions changed, it was restored via labour and magic by his descendants to fit in with contemporary designs. (You can see the Victorians "improving" it like that...)
He's carrying a batch of scrolls because there are young witches and wizards to be educated! If you would like to read more (and there is a lot more), go here: sta.sh/0atolrlj8ih The photo isn't as high quality as I'd like along the bottom, but the picture was also drawn in much smaller scale than the fullview.
How many snakes can you find in the picture?
Other Historical Founders
I am glad you found this all so interesting!
For inspiration on jewellery, I would first suggest you look at some of the metalwork from the Sutton Hoo burial. It's a bit before this period, but is the burial of an East Anglian king. To see designs and a few great examples of Anglo-Saxon metal work (including jewellery), I'd suggest you start here: en.wikipedia.org/wiki/Anglo-Sa… I'm not 100% sure on which jewellery men wore versus women. I didn't see any example of men wearing earrings, and I'm not sure about neckwear since their collarbones were always covered by their cloaks and tunics in the images I saw. They might have worn chains on their necks, but I'm not sure. Rings are definitely fair game. You might find more information if you dig around this source: www.regia.org/members/handbook…
As for lockets, I did not find any 10th century examples. If you check out the 'locket' section on my stash you will find a link or two with pictures of later medieval lockets, which I used for inspiration.
Hope that helps! And as for snakes, there might be one or two more...