Cheapest priced copy (Future Sight): $0.35 at Card Kingdom, near mint, their price list as of .
Riddle of Lightning 
Instant · Uncommon · Journey into Nyx · #107
Choose any target. Scry 3, then reveal the top card of your library. Riddle of Lightning deals damage equal to that card's mana value to that permanent or player.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
4 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Future Sight | 105 | Common | $0.35 | $0.49 |
| Jumpstart | 359 | Uncommon | $0.35 | — |
| Journey into Nyx | 107 | Uncommon | $0.35 | $0.49 |
| Time Spiral Remastered | 183 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 19 March 2021If the chosen target is an illegal target by the time Riddle of Lightning tries to resolve, the spell doesn't resolve. You don't scry 3 or reveal any card.
- 19 March 2021If a card in a player's library has {X} in its mana cost, X is considered to be 0.