Cheapest priced copy (The List): $0.49 at Card Kingdom, near mint, their price list as of .
Whispersteel Dagger 
Artifact — Equipment · Rare · Zendikar Rising Commander · #5
Equipped creature gets +2/+0.
Whenever equipped creature deals combat damage to a player, you may cast a creature spell from that player's graveyard this turn, and you may spend mana as though it were mana of any color to cast that spell.
Equip {3}
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The List | ZNC-5 | Rare | $0.49 | — |
| Zendikar Rising Commander | 5 | Rare | $0.49 | — |
4 rulings from Wizards of the Coast
- 25 September 2020You don’t choose a card to cast as Whispersteel Dagger’s ability resolves. You simply get a permission to cast one later.
- 25 September 2020You’ll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.
- 25 September 2020In a multiplayer game, if a player leaves the game, all cards that player owns leave as well. If you leave the game, any spells and/or permanents you control from Whispersteel Dagger’s ability are exiled.
- 25 September 2020You must follow the normal timing permissions and restrictions for the spell you wish to cast.