Island of Shadow
Source: Yaen

1. Location

2. Island Heart acquisition

  • The Island Heart can be obtained by purchasing it with a quest item dropped from the last tower.
  • The merchant appears after you finish the purple quest on the island.

2.1. Quest Details

  • The quest is fairly straightforward. Kill stuff.
  • Part of the quest will require you to talk to some dead guys on the floor.
  • The last part of the quest requires you to go to the 5th floor to kill these bosses pictured below. 
  • They have a chance of dropping the item you need to complete the quest. They spawn every hour.