Truest Polymorph

This spell is commonly found being sold as a scroll in shady markets as a "discount" version of a True Polymorph spell. The seller usually claims it's a one time use and self-targeted version that otherwise has all the same functions and limitations as the full version of the spell. While some casters eventually become able to learn and cast this spell as part of their repertoire, anyone can cast this spell from a scroll regardless of class or spellcasting ability, no check required.

While Truest Polymorph is very powerful in it's own right, the advertised effect is not the real effect. Casting this spell instead instantly and irreversibly transforms the caster into its truest form, that is, the physical form most accurately representing the deepest and most fundamental facets of their personality. A courageous and true of heart paladin may suddenly find himself learning to use claws instead of a sword in the body of a huge and powerful lion, while a sneaky silver-tongued con artist may find it much harder to charm his marks when he's hissing from a snake mouth.

More extreme effects may even be possible depending on the personality. The elderly former captain of the town guard may find a new purpose as a sentient enchanted blade passed down to future generations, or the seemingly stone-skinned barbarian may find his new golem form capable of withstanding even greater punishment.

The caster is limited by the nature of it's new form and replaces it's racial features and physical ability scores with that of the new form, but retains it's mental ability scores and all of its other features it is still physically capable of using including class features and the ability to speak and cast spells. The caster's gear falls to the ground during the change and can only be re-equipped if the new form is capable of using it.

The caster's form does not revert when they drop to zero hit points, cannot be dispelled, and their very soul changes to match that of the new form. Because of this, no magic, save for a wish spell or divine intervention, can reverse this change as there is no longer a previous form to revert to. The caster is not considered a shape-changer or a transformed creature, they simply are the new form as if they had been born that way. This spell does not prevent future shape-changing magic from functioning, but the normal form the caster reverts to when the magic ends is the form granted by this spell.