Cheapest priced copy (Theros Beyond Death): $0.35 at Card Kingdom, near mint, their price list as of .
Arena Trickster 
Creature — Human Shaman · 3/3 · Common · Theros Beyond Death · #126
Whenever you cast your first spell during each opponent's turn, put a +1/+1 counter on this creature.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros Beyond Death | 126 | Common | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 24 January 2020In a Two-Headed Giant game, this ability triggers no more than once during each opposing team’s turn.
- 24 January 2020An ability that triggers when you cast a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
- 24 January 2020If you have more than one opponent, this ability can trigger once during each of those opponents’ turns.
- 24 January 2020This ability triggers only on your very first spell during an opponent’s turn, not the first spell after the card is on the battlefield. If you cast a spell before it’s on the battlefield (including if you cast this card somehow during an opponent’s turn), the ability won’t trigger.