In southeastern part of the main island, just south from Kvatch city and its wayshrine.
If you start ascending toward Kvatch you’ll spot some ruins to your right. There is a half ruined tower with a couple of walls hanging on. You might not be able to see the shard’s glow from a distance, but if you go down into the ruins you’ll surely see it. There are some Minotaurs in the area, but if you come in sneaking, you can avoid them until you pick up the skyshard.
East from it there is an area of interest – Varen’s Wall Gatehouse, while going north you can find Kvatch’s Arena, one of the most fun group boss fights.