Holy Water can be made by filtering fresh water through 10 layers of moon moss, then casting bless on the water. Each casting of bless can create about 1 barrell of holy water.
Throw it on the Undead to damage them or drive them away.
note: Salt water will work but has an unfortunate side effect of being a fast acting laxative. While there are occasions when you may want that side effect, it’s generally detrimental when in combat.