Cheapest priced copy (Star Trek Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Promise of Loyalty 
Sorcery · Rare · March of the Machine Commander · #200
Each player puts a vow counter on a creature they control and sacrifices the rest. Each of those creatures can't attack you or planeswalkers you control for as long as it has a vow counter on it.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
13 printings · $0.49 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Bloomburrow Commander | 148 | Rare | $0.59 | — |
| Commander 2021 | 21 | Rare | $0.59 | — |
| Commander 2021 | 348 | Rare | $2.29 | — |
| Commander Masters | 833 | Rare | $0.59 | — |
| Final Fantasy Commander | 249 | Rare | $0.69 | $4.49 |
| Murders at Karlov Manor Commander | 79 | Rare | $0.59 | — |
| March of the Machine Commander | 200 | Rare | $0.59 | — |
| Marvel Super Heroes Commander | 142 | Rare | $0.59 | $0.99 |
| Marvel Super Heroes Commander | 319 | Rare | $1.29 | — |
| The List | BLC-148 | Rare | — | — |
| Secrets of Strixhaven Commander | 161 | Rare | $0.59 | — |
| Star Trek Commander | 234 | Rare | $0.99 | $0.99 |
| Star Trek Commander | 426 | Rare | $0.49 | $0.99 |
3 rulings from Wizards of the Coast
- 16 April 2021If the vow counter is removed from the creature, it can attack as normal.
- 16 April 2021If the vow counter is moved to another creature, it won't prevent the second creature from attacking normally.
- 16 April 2021Each player, including you, must put a vow counter on a creature they control if able. They can't choose to opt out and sacrifice all of their creatures. The creature you put a vow counter on will remember its promise if another player gains control of it.