Cheapest priced copy (Gatecrash): $0.35 at Card Kingdom, near mint, their price list as of .
Mark for Death 
Sorcery · Uncommon · Gatecrash · #99
Target creature an opponent controls blocks this turn if able. Untap that creature. Other creatures that player controls can't block this turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Gatecrash | 99 | Uncommon | $0.35 | $0.49 |
3 rulings from Wizards of the Coast
- 24 January 2013If there are any costs required for that creature to block (such as the one imposed by War Cadence), the creature’s controller isn’t required to pay those costs. In that case, the requirement to block would do nothing.
- 24 January 2013That creature’s controller still decides which attacking creature it blocks.
- 24 January 2013If the creature can’t block (perhaps because it has become tapped or it doesn’t have flying and all attacking creatures do), then the requirement to block does nothing.