Cheapest priced copy (Bloomburrow Commander): $1.29 at Card Kingdom, near mint, their price list as of .
Beast Within 
Instant · Uncommon · Duel Decks: Heroes vs. Monsters · #69
Destroy target permanent. Its controller creates a 3/3 green Beast creature token.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
40 printings · $1.29 to $99.99 at Card Kingdom
One ruling from Wizards of the Coast
- 19 March 2021If the target permanent is an illegal target by the time Beast Within tries to resolve, the spell won't resolve. No player creates a Beast token. If the target is legal but not destroyed (most likely because it has indestructible), its controller does create a Beast token.