Cheapest priced copy (Duel Decks: Nissa vs. Ob Nixilis): $0.35 at Card Kingdom, near mint, their price list as of .
Shadows of the Past 
Enchantment · Uncommon · Duel Decks: Nissa vs. Ob Nixilis · #60
Whenever a creature dies, scry 1. (Look at the top card of your library. You may put that card on the bottom.)
{4}{B}: Each opponent loses 2 life and you gain 2 life. Activate only if there are four or more creature cards in your graveyard.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Nissa vs. Ob Nixilis | 60 | Uncommon | $0.35 | — |
| Magic Origins | 118 | Uncommon | $0.35 | $2.49 |
One ruling from Wizards of the Coast
- 22 June 2015Once you legally activate the last ability, it doesn’t matter how many creature cards are in your graveyard as it resolves.