Void Essence void_essence

What it is for

Loot: exists to be sold or used in a recipe.

Sells to an NPC for about 3,680 gold. Weighs 2 of your carry capacity — and weight is what stops drops once the bag fills up.

The best source is Primordial Echo, at 66% per kill. Another 3 monsters also drop it. You can farm it at Void Rift.