Stratholme Holy Water

Stratholme Holy Water is a stackable item that when used does 438 to 562 damage to Undead within a 10 yard radius. It is used in two quests: and.

While its usefulness in regular Undead instances like Stratholme cannot be denied, it really shines as an assist during the third phase of the Nefarian fight, in which upwards of forty undead constructs spawn and rush the group. The Water is generally used by the warriors who run out to intercept the constructs, as they are apt to have the most constructs around them, so they get the most benefit out of them for the raid.

Source
It is only obtainable in Stratholme as the name may imply. It can be found inside supply crates scattered throughout the instance. Boxes labled Stratholme Supply Crates always contain Stratholme Holy Water, whereas the boxes labeled Supply Crates are often trapped. Item also sometimes is droped by Undead Postman.