Cheapest priced copy (Battlebond): $2.99 at Card Kingdom, near mint, their price list as of .
Sentinel Tower 
Artifact · Rare · Battlebond · #79
Whenever an instant or sorcery spell is cast during your turn, this artifact deals damage to any target equal to 1 plus the number of instant and sorcery spells cast before that spell this turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $2.99 to $15.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Battlebond | 79 | Rare | $2.99 | $15.99 |
4 rulings from Wizards of the Coast
- 8 June 2018For example, the first instant or sorcery spell cast during your turn causes Sentinel Tower to deal 1 damage; the second causes it to deal 2 damage, and so on.
- 8 June 2018You choose the target for Sentinel Tower’s ability, regardless of who cast the spell that caused it to trigger.
- 8 June 2018Sentinel Tower’s ability counts all instant and sorcery spells cast in the turn before the spell that caused it to trigger, even if those spells are still on the stack or have been countered.
- 8 June 2018Sentinel Tower’s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.