Cheapest priced copy (Alliances): $0.49 at Card Kingdom, near mint, their price list as of .
Keeper of Tresserhorn 
Creature — Avatar · 6/6 · Rare · Alliances · #52
Whenever this creature attacks and isn't blocked, it assigns no combat damage this turn and defending player loses 2 life.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alliances | 52 | Rare | $0.49 | — |
2 rulings from Wizards of the Coast
- 4 October 2004The ability triggers on declaration of blockers if the criteria is met.
- 15 April 2013An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.