Cheapest priced copy (Commander Masters): $0.35 at Card Kingdom, near mint, their price list as of .
Thorn of the Black Rose 
Creature — Human Assassin · 1/3 · Common · Commander Masters · #190
Deathtouch
When this creature enters, you become the monarch.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
4 printings · $0.35 to $3.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 190 | Common | $0.35 | $1.49 |
| Commander Legends | 154 | Common | $0.35 | $0.49 |
| Conspiracy: Take the Crown | 48 | Common | $0.49 | $3.99 |
| The List | CN2-48 | Common | $0.35 | — |
9 rulings from Wizards of the Coast
- 23 August 2016Abilities that trigger whenever you "become the monarch" trigger only if you aren't already the monarch. For example, if you are already the monarch as Custodi Lich enters the battlefield, its last ability won't trigger.
- 23 August 2016The game starts with no monarch. Once an effect makes one player the monarch, the game will have exactly one monarch from that point forward.
- 23 August 2016If the triggered ability that causes the monarch to draw a card goes on the stack, and a different player becomes the monarch before that ability resolves, the first player will still draw the card.
- 23 August 2016Being the monarch carries two inherent triggered abilities. "At the beginning of the monarch's end step, that player draws a card" and "Whenever a creature deals combat damage to the monarch, its controller becomes the monarch."
- 10 November 2020There are two inherent triggered abilities associated with being the monarch. These triggered abilities have no source and are controlled by the player who was the monarch at the time the abilities triggered. The full texts of these abilities are "At the beginning of the monarch's end step, that player draws a card" and "Whenever a creature deals combat damage to the monarch, its controller becomes the monarch."
- 10 November 2020If the triggered ability that causes the monarch to draw a card goes on the stack and a different player becomes the monarch before that ability resolves, the first player will still draw the card.
- 10 November 2020If the monarch leaves the game during another player's turn, that player becomes the monarch. If the monarch leaves the game during their turn, the next player in turn order becomes the monarch.
- 10 November 2020The game starts with no monarch. Once an effect makes one player the monarch, the game will have exactly one monarch from that point forward. As a player becomes the monarch, the current monarch (if any) ceases being the monarch.
- 10 November 2020If combat damage dealt to the monarch causes that player to lose the game, the triggered ability that causes the controller of the attacking creature to become the monarch doesn't resolve. In most cases, the controller of the attacking creature will still become the monarch as it is likely their turn.