Obtained from a bag dropped by the Slow Turtle Captain NPC. This NPC has a lot of HP and takes some time to kill. Ideally you will want to team up to kill him just to burn through the HP faster.

May take sometime to earn it, I have 15+ kills on him without the drop.

If you just want to be demotivate yourself, at the time of writing this, only 0.05% of Korean players have this island token.



Source: https://www.mgx.kr/lostark/statistic/

I advise you to simply come and kill alternating while you are in PVP queues or late end game.

EDIT: By the way, it seems that you get more item drop in proportion to the damage you do to him whether you are in a group or not. You can test it. I don't know if it will be the same for Setino's Secret Bag.