One of the first side quests in Temtem is called Gone Sillaro, in which you need to find the missing pendant.
How to start a quest?
Complete the game's prologue until you gain control of the hero outside the professor's house. Walk east from the house along the coastline, where you will see a woman named Kl'h. Talk to her, and during the conversation she will mention the pendant that was lost on the Sillaro River.
Where can I find the pendant?
Once you have your surfboard, take a trip down the Sillaro River to find the pendant. The exact location is shown in the screenshot above. This beach is located east of the docks at Brical de Mar. Find sparks in the sand and interact with them to get a pendant.
Return to Zadar, talk to Claire and return the pendant to her - as a reward you will receive item TC001: Tsunami. This is a fighting technique that can be taught to your Temtem.