What is the singular possessive form of the word Hades?

The noun 'Hades' (in ancient Greek mythology) or hades (in modern English) is a singular noun as a Word for the underground abode of the dead (in Greek mythology) or a synonym for hell, the place of eternal punishment for the wicked after death, presided over by Satan (in modern English).

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: Hades'

Add an apostrophe s ('s) after the existing s at the end of the Word: hades's

Note: The form 'hade' is a noun, a Word (in geology) for the inclination from the vertical of a mineral vein or fault; and a verb meaning to incline from the vertical.