In certain Christian traditions, holy water is a mixture of salt and water that has been blessed by a priest. Witches, vampires, and other nasty evil creatures were considered violently allergic to holy water. In the middle ages, due to the demand, there was even a delivery service for holy water in cities, like the milkman in the 20th century.
phantom has its roots in Greek and came to Middle English through Old French
ghost comes from proto-Germanic and survives in most Germanic languages. In Flemish, for example, we have no commonly used equivalent to phantom (there's one in the dictionary but nobody uses it)
Etymologically, these represent two very different concepts. The Greek φάντασμα ...