Cheapest priced copy (Guilds of Ravnica): $0.35 at Card Kingdom, near mint, their price list as of .
Necrotic Wound 
Instant · Uncommon · Foundations Jumpstart · #470
Undergrowth — Target creature gets -X/-X until end of turn, where X is the number of creature cards in your graveyard. If that creature would die this turn, exile it instead.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 79 | Uncommon | $0.35 | $0.49 |
| Jumpstart 2022 | 447 | Uncommon | $0.35 | — |
| Foundations Jumpstart | 470 | Uncommon | $0.35 | — |
| Guilds of Ravnica Promos | 79 | Uncommon | — | $0.49 |
5 rulings from Wizards of the Coast
- 5 October 2018If you have no creature cards in your graveyard, the target creature gets -0/-0, and it will still be exiled if it would die this turn.
- 5 October 2018Necrotic Wound's replacement effect will exile the target creature if it would die this turn for any reason, not just immediately after Necrotic Wound resolves.
- 5 October 2018The value of X is determined only as the undergrowth ability resolves. If the number of creature cards in your graveyard changes later in the turn, the target creature is unaffected.
- 5 October 2018Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.
- 5 October 2018Because tokens aren't cards, they never count for undergrowth abilities.