The easiest way to get to the Ring of Stone is to start from the entrance of the New Camp, where you and Mordrag parted ways after Thorus' Quest. From there, there should be a mountain trail leading off beyond the entrance gate, going over the right side of the nearby lake, and around behind the rocks of the beach on the other side. If you proceed along this path, you will find some Snappers living in a cave just off the trail at one point, so I suggest taking caution. On the other hand, you can swim across the lake to the beach area, but you have to watch-out for the Lurker(s), the Bloodflies, and the park of Biters on the other side. The benefit of the mountain trail is that you can approach Milton before you stumble onto the area where the Focus Stone is.
Oh, yes: the Ring of Stone is represented by the lower-left point of the pentagram on the map Saturas will give you.
In either case, somewhere close to the mountains on the other side, you will encounter Milton of the Fire Mages slaying a Blood Hound. You can help or let him finish it off on his own. Talking to him, he will mention a crypt nearby that he was going to explore, and a protected Orc Talisman, as well as a curse that has trapped one of the last people who tried to pilfer the talisman as an invincible undead zombie called the Warden. If you agree to go and explore the crypt with him, Milton will give you a scroll of Death to the Undead. Death to the Undead is the ONLY way to kill one of the creatures in the crypt.
Follow Milton over to the altar of stone and where the entrance that leads down to the crypt is located. Milton will tell you to lead on. First, you will fight the many many skeletons of the crypt, but you can make this easier by drawing them out of the crypt and running around the entrance until they lose sight of you. You can walk right up behind them and get a few good swipes on one before they all turn to chase you again. Rinse and repeat. No one ever said that the undead were smart. Milton will also help, of course, but don't expect him to hunt them out if there's all outside the entrance and he's waiting inside. No one ever said that Milton was smart. Once the skeletons are either killed, or moved out of the way, go after the Warden with the scroll of Death to the Undead. The Warden has the Orc Talisman desired by Milton and he will retrieve it from you after you pick it up yourself; the chest next to the Warden has the Focus Stone in it.
If you're not here with Milton, just aggro the Warden out of the crypt (slower than molasses) and grab the items in the chest while he's away.