Saturas will instruct you to find the other four Focus Stones, as all five will be needed to bring down the barrier. He'll give you a map of the previous locations of the Focus Stones - where they were initially set to create the barrier - and theorize that they must still be somewhere nearby. For whatever reason, there is a nifty irregular, inverted pentagram drawn on the map indicating where the Stones were by its five points. Also, Saturas will give you a scroll that will teleport you directly to his chamber in the Water Mages's grotto (the scroll is worth no ore). And off you go.
Whenever you return to Saturas's chamber and you have used the teleport scroll, he will give you another. You can return to Saturas for every stone, or collect several stones before returning. After you deliver the last stone, Aid of the Magicians of Fire will start in which you'll be given a rune (no magic circle required) that does the same thing, so don't try saving that teleport scroll for later.
For simplicity's sake, as far as navigation is concerned, the upside-down pentagram on the map you were given is useful as a guide:
- Top-left vertex: Focus in Troll Canyon
- Top-right vertex: Focus in the Monastery Ruins
- Bottom-left vertex: Focus under the Ring of Stone
- Bottom-right vertex: You collected this one already ...
- Bottom-most vertex: Focus in the Mountain Fort
All Focus Stones are in place prior to the triggering of this mission AND can be retrieved at any time, usually via the use of some Telekinesis scrolls, supplementing what scrolls you'd get at the site, during this mission (see specific Focus Stone missions for details).