Cheapest priced copy (Commander Masters): $0.79 at Card Kingdom, near mint, their price list as of .
Demon of Fate's Design 
Enchantment Creature — Demon · 6/6 · Rare · Duskmourn: House of Horror Commander · #137
Flying, trample
Once during each of your turns, you may cast an enchantment spell by paying life equal to its mana value rather than paying its mana cost.
{2}{B}, Sacrifice another enchantment: This creature gets +X/+0 until end of turn, where X is the sacrificed enchantment's mana value.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
3 printings · $0.79 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 731 | Rare | $0.79 | — |
| Commander Masters | 762 | Rare | $1.49 | $4.49 |
| Duskmourn: House of Horror Commander | 137 | Rare | $0.79 | — |
2 rulings from Wizards of the Coast
- 28 July 2023If you cast a spell with {X} in its mana cost this way, the only legal choice for X is 0.
- 28 July 2023If you cast a spell for another cost "rather than paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.