Cheapest priced copy (Duel Decks: Knights vs. Dragons): $0.49 at Card Kingdom, near mint, their price list as of .
Mordant Dragon 
Creature — Dragon · 5/5 · Rare · Worldwake · #85
Flying
{1}{R}: This creature gets +1/+0 until end of turn.
Whenever this creature deals combat damage to a player, you may have it deal that much damage to target creature that player controls.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Knights vs. Dragons | 58 | Rare | $0.49 | — |
| Starter Commander Decks | 152 | Rare | $0.49 | — |
| Worldwake | 85 | Rare | $0.49 | $0.49 |