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