Cheapest priced copy (Phyrexia: All Will Be One): $0.59 at Card Kingdom, near mint, their price list as of .
Black Sun's Twilight 
Instant · Rare · Phyrexia: All Will Be One Promos · #84p
Up to one target creature gets -X/-X until end of turn. If X is 5 or more, return a creature card with mana value X or less from your graveyard to the battlefield tapped.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $2.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Phyrexia: All Will Be One | 84 | Rare | $0.59 | $0.49 |
| Phyrexia: All Will Be One | 383 | Rare | $0.99 | $1.49 |
| Phyrexia: All Will Be One Promos | 84p | Rare | $1.49 | $0.99 |
| Phyrexia: All Will Be One Promos | 84s | Rare | — | $2.99 |
2 rulings from Wizards of the Coast
- 4 February 2023You can cast Black Sun's Twilight without a target (presumably with X equal to 5 or more) just to return a creature card. However, if you do choose a target, and that target is illegal at the time the spell tries to resolve, the spell won't resolve and none of its effects will happen. You won't get to return a creature card to the battlefield.
- 4 February 2023The creature card you return, if any, isn't a target of Black Sun's Twilight. You choose it as Black Sun's Twilight resolves if X is 5 or more.