Commune divination spell mentions "unholy water" as its material component:
Components: V, S, M (incense and a vial of holy or unholy water)
This is the only "unholy water" mentioning I've found in the PHB so far. All other chapters mention holy water only:
Holy Water. As an action, you can splash the contents of this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. In either case, make a ranged attack against a target creature, treating the holy water as an improvised weapon. If the target is a fiend or undead, it takes 2d6 radiant damage. A cleric or paladin may create holy water by performing a special ritual. The ritual takes 1 hour to perform, uses 25 gp worth of powdered silver, and requires the caster to expend a 1st-level spell slot.
Is "unholy water" a vestige from the 3.5 edition that is supposed to be removed (or rewritten) in 5e? Is it just an "evil" equivalent of holy water that can be used interchangeably? Or does it have special meaning in 5e?