A new way to acquire a random Legendary pattern has been added in Patch 9.2! In Patch 9.2, Bonesmith Heirmir now also sells Chronicle of Lost Memories for 600 Soul Cinders. This item will give your character a random Legendary Pattern that you haven't obtained.
If your alt is short on Soul Cinders, don't forget that you'll be able to transfer Soul Cinders between characters using Packaged Soul Cinders. This pattern will be a great supplement for trying to get a Legendary Pattern that drops from Castle Nathria or another hard to get Pattern.
You can buy this version of Chronicle of Lost Memories alongside the other ways to get it — from the Anima Conductor for 35 Grateful Offering and from Korthia for 2,000 Cataloged Research. If you're looking for a specific drop, don't forget to learn as many patterns as you can before using this item.
Read more on wowhead.com