Recipe: Transmute Undeath to Water

Transmute: Undeath to Water is a sub-skill of Alchemy which allows the Alchemist to change Essence of Undeath into Essence of Water. In order to perform the transformation, the alchemist must have a Philosopher's Stone and a skill level of 275.

Transmuting essences will trigger a 24 hour transmute cooldown.

Source
The recipe is a rare, random drop from various high-level mobs throughout the lands.