Cheapest priced copy (Portal Three Kingdoms): $32.99 at Card Kingdom, near mint, their price list as of .
Heavy Fog 
Instant · Uncommon · Portal Three Kingdoms · #136
Cast this spell only during the declare attackers step and only if you've been attacked this step.
Prevent all damage that would be dealt to you this turn by attacking creatures.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $32.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Portal Three Kingdoms | 136 | Uncommon | $32.99 | — |
3 rulings from Wizards of the Coast
- 1 October 2009If all the attacking creatures attack your planeswalkers, you can’t cast Heavy Fog. To cast it, a creature needs to have attacked _you_.
- 1 October 2009Heavy Fog prevents only damage that would be dealt to you by attacking creatures. It won’t prevent damage attacking creatures would deal to creatures or planeswalkers you control.
- 1 October 2009Heavy Fog prevents all damage attacking creatures would deal to you this turn, not just combat damage. Noncombat damage from those creatures is prevented only if they’re still attacking at the time that damage would be dealt or, if the creature has left the battlefield by then, if it was an attacking creature at the time it left.