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So, True Polymorph can be used to turn creatures into objects, like lumps of adamantine.
Creature into Object: If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form. The creature's Statistics become those of the object, and the creature has no memory of time spent in this form, after the spell ends and it returns to its normal form.
So, what keeps players from reaching level 17 and holding back for a month to turn a bunch of fish into Vorpal weapons, Belts of Giant Strength, Armor of Invulnerability, etc? Items are, as far as I understand, objects.
I had the notion that Wish was on another league, but other level 9 spells would only break combat for a few rounds. True Polymorph seems to stack its effects to the point where the party gets it and, after a month, has doubled its power, both in and out of combat.