Cheapest priced copy (Ikoria: Lair of Behemoths): $0.49 at Card Kingdom, near mint, their price list as of .
Mythos of Nethroi 
Instant · Rare · Ikoria: Lair of Behemoths · #323
Destroy target nonland permanent if it's a creature or if {G}{W} was spent to cast this spell.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $3.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ikoria: Lair of Behemoths | 97 | Rare | $0.49 | $0.49 |
| Ikoria: Lair of Behemoths | 323 | Rare | $0.99 | $3.49 |
| Ikoria: Lair of Behemoths Promos | 97p | Rare | $0.99 | $1.49 |
| Ikoria: Lair of Behemoths Promos | 97s | Rare | — | $0.69 |
5 rulings from Wizards of the Coast
- 17 April 2020Mythos of Nethroi can target any nonland permanent. Determine as the spell resolves whether the permanent is destroyed, either because it’s a creature at that time or because {G}{W} was spent to cast the spell.
- 17 April 2020If an effect copies the Mythos spell, no mana was spent to cast the copy, so the copy won’t receive the bonus.
- 17 April 2020If an effect allows you to cast a spell without paying its mana cost, you can’t choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can’t waive a cost reduction unless that effect says you may.
- 17 April 2020The ability checks what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn’t otherwise spend, but it doesn’t change what mana you spent to cast the spell.
- 17 April 2020The abilities of the Mythos check what colors of mana were spent to cast the spell. It’s not an alternative cost to cast the spell.