Fiend Arena Oversoul
As the Adamantoise has high Defense and HP, it is suggested to slowly take it down with powerful abilities to defeat it. Beware when Oversouled, as its ability Breath of the Gorgon will inflict major Fire damage and Petrification on the whole party.
In the Fiend Tale endings, one notable Adamantoise is created from the combined souls of a seafaring crew who were killed by Sin while on route back to Kilika. Its form being a reflection of their ship, the Adamantoise, the fiend only departed to the Farplane once it fulfilled its component souls' desire to return to their homeland.
Adamantoise is a portmanteau of "adamantite" and "tortoise".
Adamantite and similar words including Adamant and Adamantine are used to refer to any especially hard substance, whether composed of diamond, some other gemstone, or some type of metal.