Soul cinders are the newest Torghast currency, added in patch 9.1. You can get it from layers 8 to 16. They are required to craft rank 5 and 6 legendary items. Needless to say you’ll need a lot of it, especially if you like both PvP and PvE content. We can get you whatever amount of Soul Cinders you need in not time.
ETA: 1500 Soul cinders in about 2 hours.
Soul cinders boost rewards: