Thorn is the name of the dormant volcano that's responsible for Sehto's existence and all of its landmass.
It was revered as a minor deity by the Kyza, the first colonists and thus 'native' inhabitants of Sehto. They believed it judged their souls and brought natural disaster upon those who would attempt to upset the balance with nature, but later dropped their belief in it once the Legendaries had their attention.
Unbeknownst to the Kyza, Thorn is an actual metaphysical phenomenon, manifesting in ways they did not account for mythologically. Thorn possesses a spirit - something best considered a soul.
The spirit was bound in part with its avatar, a mortal made immortal, a sapient creature capable of understanding the world in a way the mountain itself could not, given a mark (an obvious outward sign of its alien nature) and powers beyond comprehension. The avatar's immortality can easily find an artificial end, though Thorn's spirit simply incarnates another if this occurs.
Some part of the spirit would always rest within Thorn itself, and it's conceivable that the avatar of Thorn could draw upon the knowledge of Thorn (most notably, its omnipresence) if it was allowed to live long enough to achieve enlightenment. Equally, Thorn does not possess the knowledge of its incarnations, more force of nature than guided process of thought, but a feedback and learning process (then encoded into the structure of the island itself) is theoretically possible. Deliberate terraforming is probably the most straightforward way to communicate with Thorn's non-incarnated self.
See also
- Jagdish Tsukinaka, current spirit of Thorn
- Abilities of a spirit of Thorn (as per Jagdish's incarnation)