Recipe: Transmute Water to Undeath

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

Tools
Philosopher's Stone

Transmuting essences will trigger a 24-hour transmute cooldown.

Source
The Recipe: Transmute Water to Undeath is a rare, random drop from various high-level mobs throughout the lands, although its use is subject to question, since Essence of Undeath is considerably more common (and hence less valuable) than Essence of Water.