Cheapest priced copy (Final Fantasy Commander): $1.29 at Card Kingdom, near mint, their price list as of .
Conqueror's Flail 
Artifact — Equipment · Rare · Secret Lair Drop · #1571
Equipped creature gets +1/+1 for each color among permanents you control.
As long as this Equipment is attached to a creature, your opponents can't cast spells during your turn.
Equip {2}
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
6 printings · $1.29 to $16.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Double Masters 2022 | 302 | Rare | $3.99 | $6.99 |
| Double Masters 2022 | 557 | Rare | — | — |
| Commander 2016 | 53 | Rare | $4.49 | — |
| Final Fantasy Commander | 340 | Rare | $1.29 | $4.49 |
| Teenage Mutant Ninja Turtles Source Material (Ronin's Arsenal) | 15 | Mythic | $1.99 | $4.99 |
| Secret Lair Drop | 1571 | Rare | $13.99 | $16.99 |
3 rulings from Wizards of the Coast
- 8 November 2016The five colors are white, blue, black, red, and green. Conqueror's Flail can't give a creature more than +5/+5. (Gold, artifact, and colorless aren't colors.)
- 8 November 2016If Conquerer's Flail is attached to a creature you control and you activate its equip ability targeting another creature you control, there's no time in between activating that ability and the resolution of that ability during which it's not attached to a creature. Assuming it's your turn, your opponents won't be able to cast spells.
- 8 November 2016The first ability of Conqueror's Flail constantly updates as the number of colors among permanents you control changes. If the number of colors decreases and the damage that was marked on the equipped creature earlier in the turn is now lethal damage, that creature is destroyed. Similarly, if its toughness becomes 0, that creature is put into its owner's graveyard.