In Lost Ark, Brealeos is a World Boss that only appears at certain times of day once every few days. Locate this boss, then engage it in battle; if you win, it will drop an item necessary to complete your set of Island Souls. You may be at a loss as to where this boss can be found and at what times if you've already looked on every continent. The location of Brealeos in Lost Ark is detailed here.
Instructions on Tracking Down Brealeos, the World Leader's Boss in the Ark
Frostfire Island, not far from Shushire, is where Brealeos reproduces. This island can only be found by choosing the same grid location as Shushire's port. To make things simpler, a port icon has been placed near the region at the top of the map. Frostfire Island is located in the Cold Snaps Seas region, to the north of the port. To confront Brealeos, you'll need to sail to this location. Fortunately, the island is always accessible, and you may even place a Lost Ark gold site there!
In Lost Ark, the spawn timings and dates for each field boss are set in advance. They begin at constant intervals and repeat every few days. Take a look at this sample alarm timetable for the Brealeos field manager.
You need to have the appropriate item level in order to combat Brealeos. To be useful in combat with Brealeos, your equipment must have a minimum level of 460. If you're below this level, you can still do harm, but it will be much harder to earn contribution points.
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To get to the island if you've reached the required item level, sail north from Shushire's harbor. You can get your ship fixed up in the port if it has become damaged, giving you the chance to fix it up before setting sail again.