Cheapest priced copy (Aether Revolt): $0.35 at Card Kingdom, near mint, their price list as of .
Monstrous Onslaught 
Sorcery · Uncommon · Commander Legends · #244
Monstrous Onslaught deals X damage divided as you choose among any number of target creatures, where X is the greatest power among creatures you control as you cast this spell.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Aether Revolt | 116 | Uncommon | $0.35 | $0.79 |
| Commander Legends | 244 | Uncommon | $0.35 | $0.49 |
| Foundations Commander | 217 | Uncommon | $0.35 | — |
4 rulings from Wizards of the Coast
- 10 November 2020You divide the damage among the target creatures as you cast Monstrous Onslaught. Each target must be assigned at least 1 damage.
- 10 November 2020The value of X is determined at the time you divide damage. It won't change later, even if the greatest power among creatures you control changes.
- 10 November 2020If some of the targets become illegal, the original division of damage still applies. The damage that was divided among the illegal targets simply isn't dealt; it isn't re-assigned to remaining legal targets.
- 10 November 2020The damage is dealt by Monstrous Onslaught, not by the creature with the greatest power among creatures you control.