Cheapest priced copy (Hour of Devastation): $0.35 at Card Kingdom, near mint, their price list as of .
Riddleform 
Enchantment · Uncommon · Core Set 2021 · #64
Whenever you cast a noncreature spell, you may have this enchantment become a 3/3 Sphinx creature with flying in addition to its other types until end of turn.
{2}{U}: Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Hour of Devastation | 43 | Uncommon | $0.35 | $0.49 |
| Core Set 2021 | 64 | Uncommon | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 23 June 2020Riddleform's first ability resolves before the spell that caused it to trigger but after targets for that spell are chosen. It will resolve even if that spell is countered.
- 23 June 2020If Riddleform becomes a creature the same turn it enters the battlefield, you can't attack with it unless it gains haste.