Inside the Cultist Lair, one of the first NPCs you will encounter is the Eldran, who will ask you to find 3 Cultist Markings all around the Terrapin Island for his investigation. While the first marking is easy to find, as it's next to the Sea Traveler NPC, the other two are cleverly hidden by the developers. Here is a detailed guide to locating the 3 Cultist Markings to finish the quest easily.
How to start the Eldran Quest in Fisch

Before finding the Cult Markings, you need to start the Eldran Quest to embark on this journey. You can find Eldran on the left side of the map, near the central fishing pool, where you can fish for the Cultist Lair Fishes. Interact with him, and you can start his quest.
Where are the Cultist Markings in Fisch?
Below, you can find all three Cultist Markings in Fisch, along with the fastest way to reach them.
Cult Markings 1 Location

The first Cult Marking is at the top of the island at (52, 162, 1860). Travel straight from the merchant and walk along the terrain to reach the top side of the northern part of the island. Here, you will find a Marking on the grey stone.
Cult Markings 2 Location

This one is located at the center of the Terrapin Island (-145, 150, 1945) next to the Angler Quest NPC. Go to the center, move past the merchant, and you can find these markings on the red building.
Cult Markings 3 Location

The Final Cult Marking is the easiest to find, as it is on a tree next to the Sea Traveler NPC at the edge of the map at (155, 155, 1990). You would have already met the Sea Traveler NPC to enter the Cultist Lair in the first place.
After locating all three Cult Markings, you can return to the Cultist Lair through the hidden gateway from the Terrapin Hideway. Now, return to the Eldran to get the Eldran's Amulet Fragment, which may be used as a future part of the quest in Fisch.
For more on Fisch, check out How to enter the Cultist Lair in Fisch or Fisch Crazy Cat Lady Riddle Answers – Nico’s Yarncaster Rod