Cheapest priced copy (Murders at Karlov Manor): $0.35 at Card Kingdom, near mint, their price list as of .
Behind the Mask 
Instant · Common · Murders at Karlov Manor · #39
As an additional cost to cast this spell, you may collect evidence 6. (Exile cards with total mana value 6 or greater from your graveyard.)
Until end of turn, target artifact or creature becomes an artifact creature with base power and toughness 4/3. If evidence was collected, it has base power and toughness 1/1 until end of turn instead.
Legal inCommanderBrawlStandardPioneerModernLegacyVintagePauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Murders at Karlov Manor | 39 | Common | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 2 February 2024If Behind the Mask causes a Vehicle to become an artifact creature, it doesn't count as "crewing" that Vehicle for any ability that would trigger off of a Vehicle becoming crewed.
- 2 February 2024Once you've announced that you're casting a spell, players can't take actions until you've finished doing so. Notably, opponents can't try to remove cards from your graveyard to stop you from collecting evidence.
- 2 February 2024Behind the Mask will overwrite any previous effects that set a creature or Vehicle's power and toughness to specific numbers. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
- 2 February 2024If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.