Cheapest priced copy (Conflux): $0.35 at Card Kingdom, near mint, their price list as of .
Dragonsoul Knight 
Creature — Human Knight · 2/2 · Uncommon · Conflux · #62
First strike
{W}{U}{B}{R}{G}: Until end of turn, this creature becomes a Dragon, gets +5/+3, and gains flying and trample.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conflux | 62 | Uncommon | $0.35 | $0.49 |
| Modern Masters 2015 | 112 | Common | $0.35 | $0.49 |
| The List | MM2-112 | Common | $0.35 | — |
3 rulings from Wizards of the Coast
- 1 February 2009When Dragonsoul Knight’s activated ability resolves, Dragonsoul Knight will gain flying and trample in addition to its other abilities. However, becoming a Dragon will make it lose all other creature types. It will no longer be a Human or a Knight.
- 1 February 2009If Dragonsoul Knight gains flying after blockers have been declared, it won’t cause those blocks to change.
- 1 February 2009You can activate Dragonsoul Knight’s ability more than once during a turn. The second time it resolves, it will probably have no visible effect other than giving Dragonsoul Knight another +5/+3.