TR21 is an item in the “All machines” category (TMs and HMs). It costs ₽4000.
In-game descriptions
| Sword / Shield | An all-out attack that becomes more powerful the less HP the user has. |
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Move taught
| Sword / Shield | Reversal |
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Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h