True Polymorph

9th-level Transmutation
Casting Time: 1 action
Range: 60 feet
Components: VSM (a drop of mercury, a dollop of gum arabic, and a wisp of smoke)
Duration: (C) 1 hour

Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into a nonmagical object, or the object into a creature (the object must be neither worn nor carried by another creature). The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled.

A shapechanger can automatically succeed on any saving throws against this spell, and it has no effect on a creature with 0 hit points. An unwilling creature can make a Wisdom saving throw, and if it succeeds, it isn't affected by this spell.

Creature into Creature. If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target's (or its level, if the target doesn't have a challenge rating). The creature cannot be a specific creature, and the CR of its form cannot exceed 20. The target's game statistics, including mental ability scores, are replaced by the statistics of the new form, except for Legendary Actions, Legendary Resistance, Mythic Actions, Villain Actions, or Lair Actions. It retains its alignment and personality.

The target assumes the hit points of its new form, and when it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. As long as the excess damage doesn't reduce the creature's normal form to 0 hit points, it isn't knocked unconscious.

The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions.

The target's gear melds into the new form, and is replaced by any equipment used by the new stat block, which disappears when the spell ends. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.

Object into Creature. You can turn an object into any kind of creature, as long as the creature's size is no larger than the object's size and the creature's challenge rating is 9 or lower. The creature is friendly to you and your companions. It acts on each of your turns.

You decide what action it takes and how it moves. The DM has the creature's statistics and resolves all of its actions and movement.

If the spell becomes permanent, you no longer control the creature. It might remain friendly to you, depending on how you have treated it.

Creature into Object. If you turn a creature into an object, it transforms along with whatever it is wearing and carrying into that form, as long as the object's size is no larger than the creature's size. 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.

At Higher Levels. When you cast this spell using a spell slot of 10th level, the duration is concentration, up to 10 minutes. When you cast this spell using a spell slot of 11th level, the duration is concentration, up to 1 minute. When you cast this spell using a spell slot of 12th level, the duration is instantaneous. Additionally, the CR of an object turned into a creature increases by 1 for every slot level above 9th.

Source: PHB 283, SRD

Changelog

Altered range from 30 feet to 60 feet.
Added upcasting option.
Clarified restrictions on creatures that can be transformed into, equipment conjured by certain stat blocks, and parts of stat blocks that are not retained (Legendary Actions, Resistance, etc.) Additionally, added a CR upcast benefit to creature into object.