Here is the full list of all 40 Seers Isle achievements.
Rowan chose to spend the third night alone
Rowan stopped Connor and gave up her life doing so
Connor talked with Frater Dieter in the underworld
During the second bivouac, Rowan witnessed Connor's dream
When Jennyver went missing during the first bivouac, Rowan woke up Connor
Connor took Rowan's place
Travel to the summit with Connor
Arlyn left the island, alone
The seers responded to Arlyn
During the second bivouac, Rowan witnessed Arlyn's dream
When Jennyver went missing during the first bivouac, Rowan woke up Arlyn
Arlyn and Rowan destroyed the tree
Travel to the summit with Arlyn
Freya talked with Ingrid in the underworld
Freya left the island, alone
Freya shot Erik
Connor talked with Killian in the underworld
Freya talked with Gudrid in the underworld
The group decided to take the stairs and leave Brandon behind
During the second bivouac, Rowan witnessed Freya's dream
When Jennyver went missing during the first bivouac, Rowan woke up Freya
Connor survived
Arlyn survived
Freya and Rowan left the island, at the price of Jennyver's life
Travel to the summit with Freya
Rowan survived
Erik survived
Brandon survived
Freya survived
Duncan sacrificed himself, saving Jennyver and Erik
Rowan decided to restart the cycle twice (or more)
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 Duncan's dream
When Jennyver went missing during the first bivouac, Rowan woke up Duncan
Jennyver survived
Duncan survived
Duncan went back to his clan, alone
Travel to the summit with Duncan