Nature crystal
From Emps-World Wiki
The Nature crystal is dropped by Tormented demons with a chance of 1 in 1000 kills. It upgrades the Abyssal whip, any Donator whip, the Elemental staff or the Elemental staff to their nature elemental upgrade, consuming the crystal in the progress.
The upgrade is not reversable and turns the weapon into a more powerful but degrading state, turning into the Nature crystal or the original weapon with a 50-50 chance.