Here is the full list of all 40 Seers Isle achievements.
Connor talked with Frater Dieter in the underworld
During the second bivouac, Rowan witnessed Connor's dream
Freya talked with Gudrid in the underworld
When Jennyver went missing during the first bivouac, Rowan woke up Connor
Travel to the summit with Connor
Connor took Rowan's place
Connor talked with Killian in the underworld
Freya shot Erik
During the second bivouac, Rowan witnessed Freya's dream
Freya talked with Ingrid in the underworld
Brandon survived
When Jennyver went missing during the first bivouac, Rowan woke up Freya
Connor survived
Freya survived
Travel to the summit with Freya
Freya and Rowan left the island, at the price of Jennyver's life
The seers responded to Arlyn
When Jennyver went missing during the first bivouac, Rowan woke up Arlyn
During the second bivouac, Rowan witnessed Arlyn's dream
Rowan chose to spend the third night alone
The group decided to not to leave Brandon behind at the beach
Travel to the summit with Arlyn
Arlyn and Rowan destroyed the tree
Arlyn survived
Rowan survived
The group decided to take the stairs and leave Brandon behind
Rowan impaled Erik with Duncan's sword
Rowan decided to restart the cycle twice (or more)
Jennyver survived
When Jennyver went missing during the first bivouac, Rowan woke up Duncan
During the second bivouac, Rowan witnessed Duncan's dream
Rowan spent the third night with either Arlyn or Duncan
Travel to the summit with Duncan
Duncan went back to his clan, alone
Duncan survived
Arlyn left the island, alone
Rowan stopped Connor and gave up her life doing so
Duncan sacrificed himself, saving Jennyver and Erik
Freya left the island, alone
Erik survived