In V Rising, the official description of Primal Blood Essence is that it is an energy source distilled from blood. More than that, it is a crucial crafting material used to craft many end-game objects and complete upgrades. You'll use Primal Blood Essence to craft weapons like the Dark Silver variant, armor like the Shadowmoon set, upgrade your Castle to levels 4 and 5, and build essential workbenches like the Anvil and Ancestral Forge.