Companion's Destiny

Legacy's Fate opens with Vistamir and Emaris (the island of Valtar's leaders) discussing Randak, the unlikely protagonist who is considered an eccentric storyteller. Telling his tales at the Annual River Celebrations (held in honor of the Meander River - their only clean water supply), all but Vistamir dismissed them as fanatical musings of a senile old man.
Vistamir sought out Randak the following day, and when the storyteller floated across the river in front of the leader, Vistamir's suspicions about the old man being special were real. Randak met with Vistamir and some trusted friends the next day, when the old man then revealed the surprising secret of his life as well as theirs.
It turns out that Randak (a.k.a. Doug Parker) is a 400-year-old longevity experiment, and that all the people on Valtar were descendants of a spaceship crew from Earth that was forced to land there centuries ago. On this foreign planet the current population had forgotten their roots, so the storyteller's revelation was life changing. But the leaders and the others wanted more proof his stories were true, especially Doug's cryptic remark about something killing the island.
Doug then takes Vistamir and Emaris as well as Mystel and Kaskin (the leader's twin female partners) and the twin's father Baldor to the spaceship's well-preserved remains hidden in the island's northwestern mountains. The old man's credibility was now confirmed, and afterward he took his guests to the Meander River's source, located in a cave high up in the mountains. Doug told them his suspicions were true after testing the water here: their only fresh water supply was becoming chemically polluted and will eventually be undrinkable.
After the shock of this revelation, the companions head back to the ship where Doug instructs them on building some simple filtration units using lava rock. They spend the night there and plan another trip back to the source the next day - the extra lava rock they nee