Cheapest priced copy (Modern Horizons 3): $0.35 at Card Kingdom, near mint, their price list as of .
Signature Slam 
Instant · Uncommon · Modern Horizons 3 · #168
Put a +1/+1 counter on target creature you control, then each modified creature you control deals damage equal to its power to target creature you don't control. (Equipment, Auras you control, and counters are modifications.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Horizons 3 | 168 | Uncommon | $0.35 | $0.49 |
5 rulings from Wizards of the Coast
- 7 June 2024A creature that is equipped is considered modified no matter who controls the Equipment that's attached to it.
- 7 June 2024If the creature you control is an illegal target as Signature Slam tries to resolve but the creature you don't control is still a legal target, each modified creature you control (which may include the target creature you control if it's still on the battlefield) will still deal damage equal to its power to the creature you don't control.
- 7 June 2024An Aura controlled by another player does not cause a creature you control to be modified.
- 7 June 2024A creature with a counter on it is considered modified no matter what kind of counter it is or which player put it on that creature.
- 7 June 2024If the creature you don't control is an illegal target as Signature Slam tries to resolve but the creature you control is a legal target, you'll still put a +1/+1 counter on the target creature you control.