So we now have two Halicarnassus's, wasn't this an issue before? why is now different

[Kuwagami]

Here's what a Merriam-Webster editor has to say about pluralizing foreign words in English.

Please notice how I said the plural form doesn't exist, since it's not a word per say but a name.

[KMFDM_Kid2000]

We also have 2 Kurrea's now. The A rank hunt in Mor Dhona, and the FATE boss in the Lochs. They're both different mob species.

Everyone knows North Kurrea is best Kurrea.

[Sigiria]

Also there's the Croque-mitaine, rank S hunt target, wich is the french version of the boogeyman. But there is also the boogyman in amdapor keep hard. So in the french version we've got two completly different monsters called Croque-mitaine.
Guess sometimes the localisation team just shudders and go "meh, whatever"

[Lustaria]

I do not think that the Lady of Haukke Manor is the origin of the fairy tale on Deltascape; as the whole point of Deltascape is that FF5 itself is the fairytale. ..Although in FF5 Halicarnassus is a dude.

[TouchandFeel]

Please notice how I said the plural form doesn't exist, since it's not a word per say but a name.

A word is simply a categorization of a concept or thing in order to be able express said idea and have people understand you. A name is a word and words are the names given to the idea of a thing. Nouns are the most direct category of "words that are names", with the subcategory of proper nouns covering individual or formal names for specific things, denoted by capitalization of the first letter. Halicarnassus is an example of a proper noun.

Proper nouns follow the same rules for pluralization as regular nouns.

Whether or not there is only one of that thing in existence is irrelevant because it can still be pluralized since the concept of more than one is possible, as can be seen in the following example sentence. "Upon examination two Hollywoods were in existence, the Hollywood of the silver-screen and the darker Hollywood that was full of crime and vice." While Hollywood is a singular place, the expression of multiples of it is possible.

Also, it must be kept in mind that the "Halicarnassus" being spoken about is not the Ancient Greek city-state, but the names of multiple fictional characters existing in the game. While there is, or was, only one of the city, there is definitely more than one Halicarnassus in the context of this thread and discussion.

You are correct that the proper pluralization of "-os" in Ancient Greek would be "-oi" and that would be pertinent if we were speaking of Halicarnassus the city-state and were using the more phonetically correct spelling that ends in "-os", however the subject was the fictional characters named Halicarnassus and in this instance it would be more correct to pluralize using standard English grammar rules due to the level of separation from the source. Halakarnassoi as plural is a sound and correct argument, just not in this specific case. In addition, the Latin and Greek specialcase pluralization, for example syllabi, is used for formal or scientific settings while the standard form of pluralization, syllabuses, is used in vernacular.

Whew ... I think that is enough grammar related talk for the day, unless you starting using double negatives like "you don't know nothing". Then we have words.

[Rongway]

Please notice how I said the plural form doesn't exist, since it's not a word per say but a name.

And yet you wouldn't blink at two Bobs or three Joneses.

[Kuwagami]

And yet you wouldn't blink at two Bobs or three Joneses.

This isn't actually a thing in my language and I wasn't aware that names had a plural form in English. Heh