Cheapest priced copy (Bloomburrow): $1.49 at Card Kingdom, near mint, their price list as of .
Emberheart Challenger 
Creature — Mouse Warrior · 2/2 · Rare · Bloomburrow · #315
Haste
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Valiant — Whenever this creature becomes the target of a spell or ability you control for the first time each turn, exile the top card of your library. Until end of turn, you may play that card.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
5 printings · $1.49 to $159.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Bloomburrow | 133 | Rare | $1.49 | $1.99 |
| Bloomburrow | 315 | Rare | $2.49 | $3.49 |
| Bloomburrow Promos | 133p | Rare | $2.29 | $3.49 |
| Bloomburrow Promos | 133s | Rare | — | $1.99 |
| Secret Lair Promo | 48 | Rare | $10.99 | $159.99 |
4 rulings from Wizards of the Coast
- 26 July 2024If a spell or ability you control has one or more of its targets changed to a creature you control with a valiant ability, that ability will trigger if that creature hasn't yet been the target of a spell or ability you control this turn.
- 26 July 2024Valiant abilities will resolve before the spell or ability that caused them to trigger.
- 26 July 2024If you create a copy of a spell on the stack and target a creature you control with a valiant ability, that ability will trigger as long as the creature hasn't yet been the target of a spell or ability you control this turn.
- 26 July 2024You pay all costs and follow all timing rules for cards played this way. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.