does not receive an additional s because it is plural.
I usually turn to two sources for Grammar answers, the first being Grammar Girl, and the second being Purdue OWL.Grammar Girl
Is it Kansas's statute with an apostrophe s or Kansas' statute with just an apostrophe at the end?
Associated Press style ... recommends leaving off the extra s. Some of you have noticed that I tend to favor AP style, so you won't be surprised to learn that I prefer to leave off the extra s. Unfortunately, I have to admit that this isn't a hard-and-fast rule; it's a style issue. Other style books such as Fowler's Modern English Usage recommend adding the apostrophe s to almost all singular words that end with s.*
*The exceptions according to Fowler’s are words such as Moses and Bridges that end with an s that makes an /iz/ sound, classical names such as Zeus and Venus, and Jesus. Fowler's suggests that all of these words should end with just an apostrophe (e.g., Moses' tablets).
So, likely, that says that even using the example style of Fowler's, Hercules would not receive an additional s.
In contrast, The Purdue OWL
add 's to the singular form of the word (even if it ends in -s):
the owner's car
James's hat (James' hat is also acceptable. For plural, proper nouns that are possessive, use an apostrophe after the 's': "The Eggleses' presentation was good." The Eggleses are a husband and wife consultant team.)
... which simply states that both are acceptable.
Regardless, The x of y
, instead y's x
, is still a notable improvement for a title.
Now, whether Glories
should be used in a countable manner or not would depend upon the specific meaning
4 [countable] a special cause for pride, respect or pleasure
The temple is one of the glories of ancient Greece.
Her long black hair is her crowning glory(= most impressive feature).