Cheapest priced copy (Adventures in the Forgotten Realms): $4.49 at Card Kingdom, near mint, their price list as of .
Inferno of the Star Mounts 
Legendary Creature — Dragon · 6/6 · Mythic · Adventures in the Forgotten Realms · #293
This spell can't be countered.
Flying, haste
{R}: Inferno of the Star Mounts gets +1/+0 until end of turn. When its power becomes 20 this way, it deals 20 damage to any target.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $4.49 to $47.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 151 | Mythic | $4.49 | $4.49 |
| Adventures in the Forgotten Realms | 293 | Mythic | $6.49 | $9.99 |
| Adventures in the Forgotten Realms Promos | 151a | Mythic | — | $47.99 |
| Adventures in the Forgotten Realms Promos | 151p | Mythic | $13.99 | $16.99 |
| Adventures in the Forgotten Realms Promos | 151s | Mythic | — | $6.49 |
| Secret Lair Drop | 1015 | Mythic | — | $6.99 |
3 rulings from Wizards of the Coast
- 23 July 2021You don't choose a target when you activate the activated ability. You choose a target for the linked triggered ability only if it triggers because the power of Inferno of the Star Mounts became 20.
- 23 July 2021However, if you have a way to decrease its power, you can cause the triggered ability to trigger multiple times in a single turn.
- 23 July 2021That triggered ability triggers only if the activated ability causes the power of Inferno of the Star Mounts to become exactly 20. If its power becomes 20 in any other way, the ability won't trigger. If its power becomes 21 or greater this way, the ability won't trigger.