You will use the Peregrin-Feathered Darts to get a Submarine Ticket that you will need to the next task in the Cryptids Island. These Peregrin-Feathered Darts are much more stable and easy to use darts than the Rutty darts that you got earlier in the game, by now you are better and because of this you most likely to win the game in the first time.
How to Get the Peregrin-Feathered Darts
You will automatically get the Peregrin-Feathered Darts if you won the game of darts with guy in the pub. He will also give you a rowboat ticket that you will need in your next mission. You will also need to keep these Peregrin-Feathered Darts for the next game of darts in the pub.