Cheapest priced copy (Magic 2014): $0.49 at Card Kingdom, near mint, their price list as of .
Witchstalker 
Creature — Wolf · 3/3 · Rare · The List · #M14-202
Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Whenever an opponent casts a blue or black spell during your turn, put a +1/+1 counter on this creature.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Magic 2014 | 202 | Rare | $0.49 | $2.49 |
| The List | M14-202 | Rare | $0.69 | — |
2 rulings from Wizards of the Coast
- 1 July 2013The counter will be put on Witchstalker before the spell cast by an opponent resolves.
- 1 July 2013If an opponent casts a copy of a card that’s blue or black during your turn (as opposed to putting a copy of such a spell directly on the stack), the ability will also trigger. Check for the word “cast” in the effect that creates the copy.