Sunstone

The Incendi elementals that dwell beneath the mountains have found a way to tap into the Arbor's roots, which stretch not only across the land but also through time, and King Pyra is determined to crush the ancient tree. Twenty-two years after the defeat of the Darkwater Lords, Chonrad's widow Procella and their three children are drawn back to Heartwood to investigate the rumour of strange fires springing up across the land. Across three separate timelines, the heroes must...
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The Incendi elementals that dwell beneath the mountains have found a way to tap into the Arbor's roots, which stretch not only across the land but also through time, and King Pyra is determined to crush the ancient tree.
Twenty-two years after the defeat of the Darkwater Lords, Chonrad's widow Procella and their three children are drawn back to Heartwood to investigate the rumour of strange fires springing up across the land. Across three separate timelines, the heroes must battle to join together their ancient sunstones, to overcome the Incendi threat, and to protect the Arbor and make earth victorious once more.
"File Under": Fantasy [ Under Siege The Generation Game Incendiary Heartwood Revisited ]

"From the Trade Paperback edition."