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