To unlock this trophy / achievement you need to obtain seven jewels to upgrade the Amulet of Yggdrasil. These are obtained from different enemies and Legendary Chests throughout the game, all of which are shown below.

• Dropped by the Frost Phantom boss during The Word of Fate main quest
• Dropped by the Ogre at The Forge in the Forging Destiny main quest
• Obtained from a Legendary Chest on Alberich Island in Svartalfheim, part of The Lost Treasure favor
• Obtained from a Legendary Chest inside The Elven Sanctum in The Forbidden Sands of Alfheim
• Obtained from a Legendary Chest at The Oarsmen in Midgard
• Obtained from a Legendary Chest at Noatunn’s Garden in Vanaheim
• Obtained from a Legendary Chest in The Jungle of Vanaheim

For a quick detailed guide on where to find each of these, see our page on Where to Find Jewels of Yggdrasil to Unlock Amulet Slots.