Quote Originally Posted by rdanhenry View Post
Not clear what you are saying here, but "Abyss's" would be the preferred possessive for a single abyss. The rule is not actually arbitrary, but corresponds to when one adds a sound in speech. To be sure, there's no law to be broken here. Unlike French, English has no lawful authority to dictate language usage, but the variant that does not add the apostrophe ess just because the word ends in an ess, even though one vocally adds it, is arbitrary.
I'm saying it would be Abyss' .