Here is the solution for the Merrick riddle (SPOILER)
Sail with ur Crew to Shark Bait Cove and talk to Merrick and take the first book of him at the middle of the isle. After this u sail to M/N 13 there is an Isle which is not marked on the map, there u can climb into the crowsnest and read the second book.
Then u need to sail to the Dagger Tooth Outpost and talk to the Waitress. She is going to tell you that there is the third book in an cave. The cave is an underwater cave at S 17. To find the Cave look near the sunken ship after an stone archway.
After this you should get the message to get back to Merrick. Then you are going to become the drum.
The way to summon is 1 person needs to take the merrick shanty all the way to T26 then 4 more need to join in then your tattoos glow and it summons
Thanks for the Information @spyro-dragma
