Cheapest priced copy (Mercadian Masques): $0.39 at Card Kingdom, near mint, their price list as of .
Trap Runner 
Creature — Human Soldier · 2/3 · Uncommon · Mercadian Masques · #55
{T}: Target unblocked attacking creature becomes blocked. Activate only during combat after blockers are declared. (This ability works on creatures that can't be blocked.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.39 to $3.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mercadian Masques | 55 | Uncommon | $0.39 | $3.99 |
| The List | MMQ-55 | Uncommon | $0.39 | — |
3 rulings from Wizards of the Coast
- 4 October 2004If the ability is used on a creature with Trample, the damage still comes through.
- 4 October 2004This ability will trigger any creature’s “whenever this becomes blocked” ability.
- 20 September 2013If a turn has multiple combat phases, the ability can be activated during any of them as long as it’s after the beginning of that phase’s Declare Blockers Step.