Soul Cinders
Torghast improvements will make Legendary crafting easier in 9.1.5
Crafting Legendaries in Shadowlands can be an expensive process, and not just because you're spending gold at the Auction House to buy the proper item.
How to upgrade your Legendaries to rank 5 and 6 in patch 9.1
We've spent a lot of Shadowlands dealing with the Runecarver and getting ourselves legendary items.
Cinders, Knowledge, and Shards: These are all of the new currencies in patch 9.1
Shadowlands already has plenty of different currencies, and patch 9.1 will be adding a few more to go alongside the new raid and the Torghast rework.
You will be able to get so much Soul Ash in patch 9.1
We already noted that in patch 9.1, you'll be able to send Soul Ash to alts, meaning that you could conceivably farm Soul Ash on a bunch of your characters and funnel it towards one you want to craft a legendary for.