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