Witcher 3 Not Only Eagles Dare Walkthrough
Start Location: This quest is activated by looting a body at the HiddenTreasure location in the bay to the southwest of Svorlag on the island of Spikeroog.
You’ll find a small island in the bay just southeast of the town of Svorlag. This is Guarded by 6-7 level 15 Drowners. Wipe them all out and then loot the fellow’s body here for a Note and a Key with Ring Attached – this will activate the side-quest ‘Not Only Eagles Dare’.
Enter the water and dive under inside the investigation zone. Note that there are several Sirens swimming around the area who are likely to attack – a crossbow bolt with kill them instantly, so make sure you keep it equipped.
Activate your Witcher Senses and look for a chest beneath the water close to the island, just to its southeast. Use the Key to unlock it. This will complete ‘Not Only Eagles Dare’ and by looting the contents, you’ll be rewarded with some goodies (I received a new chest and legs).
Looting this will also end the side-quest.