Onyx Fragments are pieces of a rare Onyx Final Form card. Collecting enough of these fragments allows you to craft a complete card. You need one hundred fragments to create the card, and you can track your progress in the Upgrade menu. Once you have gathered the required number, select the option from the Upgrade menu to craft the card.
You can collect Fragments by achieving high ranks in Arena and Heroic, finding them in certain shop packs, or purchasing an Onyx Fragment Pack.
To upgrade a crafted Onyx Final Form card, you'll use fragments instead of dust. Unlike other cards, Onyx Final Forms can be upgraded up to Level 7.