Buy RS3 Zarosian essence
Zarosian essence is a light material in RuneScape 3. It is a members-only item that is tradeable and stackable, and it weighs nothing, so it takes up no extra load in your bag.
This essence is used to upgrade anima core armour found in the Heart of Gielinor. With it, you can turn regular anima core armour into its refined version. You can also use it to repair refined armour. A helm needs 1 Zarosian essence, legs need 2, and a body needs 3, along with the same amounts of Serenic, Sliskean, and Zamorakian essence. Crafting refined armour needs level 80 in the right skill and gives 500 experience per piece. This works for the Seren, Sliske, Zamorak, and Zaros armour sets.
In the game, Zarosian essence drops from Vindicta and Gorvek in the Heart of Gielinor, but the drop rate is rare. Buying it saves you from long hours of boss fights.
The essence can also be disassembled for Invention. It gives 25.5 disassembly XP and can yield Cover parts, Deflecting parts, Plated parts, Strong components, Protective components, or rare Ilujankan components.