On this page of the Baldur's Gate 3 guide, we inform how to open an elaborate reliquary found during the prologue onboard Nautiloid, a flying ship. It is not a full-fledged puzzle. However, since you don't have lockpicks, opening the chest may not be as easy.
The Elaborate Reliquary is located in one of the rooms in the northern part of the ship. This place is shown in the images. Here you can also find, e.g. a panel with three buttons, and people trapped in pods.
You can't use Thieves' Tools to open the chest, because you won't have them yet. Instead, get to the eastern room shown in the pictures. You'll find an Elaborate Key on the cleric's corpse.
Go back to the room with the chest and use the key to open it. Unfortunately, you won't find any exceptionally valuable treasures. The chest should contain gold and onyx (precious stone).