The word "eternity" is a noun and means "a span of time without end"
The word "eternality" is also, strictly speaking, a noun, but it is used as an adjective, i.e it describes a trait of the noun or phase it modifies. It means "having the trait of lasting for a span of time without end".
Therefore the sentence you cite should be "The wise men were with him for eternity". However, if the wise men hung about for a very long time and then left, you might say "The wise men were with him for AN eternity". The use of the indefinite article makes all the difference by indicating that there was, indeed, an end to the time the wise guys hung about
If the presence of the wise men were an annoyance, as the presence of wise men often is, you might say: "The eternality of the presence of the wise men was hard to bear", thereby saying in a round about way that the silly twits just wouldn't leave
Student or Learner