You can now barter Ashes of Enchantment for essences! A Northern Stronghold Essence is 2500 Ashes, Empowered Abyssal is 1500, Abyssal is 850, Empowered Shadow is 350.
If you want Malleable Essences to craft higher-tier essences, Empowered Shadow will give you one for 350 Ashes, which is the most efficient.