Speak to Jacob the Sailor in Relleka and set sail to the Lost Isles, once you are there you will need to lift a curse from the island. Go north of the island until you come across a skeleton's body, search it to find a book. Read the book and you shall get a curse with it you can see the cursed monster that attacks trespassers. Keep going east until you come across a small cave in which you will need to enter later in the quest, go back to Jacob and give the book to him, he will read it and say the monster is in the cave and you will need to defeat it with a Revenant Spear.