Polymorph

You momentarily transform another creature into another one.

An unwilling creature must make a Wisdom saving throw to avoid the effect. The spell has no effect on a shapechanger or a creature with 0 HP.

The transformation lasts for the duration, or until the target drops to 0 HP or dies. An unwilling creature can expend its action, bonus action, and movement on its turn to attempt another Wisdom saving throw using its original Wisdom score, to try and end this effect.

The new form can be any Avion, Beast, or Ichthion whose challenge rating is equal to or less than the target's (or half of the target's level, if it doesn't have CR). The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

The target assumes the HP of its new form. When it reverts to its normal form, the creature returns to the number of HP it had before it transformed. If it reverts as a result of dropping to 0 HP, 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 HP, 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. The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.

Not quite as permanent as the Hearthstone version.