Cheapest priced copy (Commander Legends: Battle for Baldur's Gate): $2.99 at Card Kingdom, near mint, their price list as of .
Reins of Power 
Instant · Rare · Commander 2015 · #103
Untap all creatures you control and all creatures target opponent controls. You and that opponent each gain control of all creatures the other controls until end of turn. Those creatures gain haste until end of turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
5 printings · $2.99 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2015 | 103 | Rare | $3.99 | — |
| Commander 2016 | 96 | Rare | $4.49 | — |
| Commander Legends: Battle for Baldur's Gate | 734 | Rare | $2.99 | — |
| Commander 2011 | 57 | Rare | $4.49 | — |
| Stronghold | 41 | Rare | $4.49 | — |
2 rulings from Wizards of the Coast
- 4 October 2004It is perfectly legal for either or both players to have zero creatures. They give control of all their zero creatures as instructed.
- 1 May 2007You only untap creatures controlled by you and the targeted opponent, not all creatures.