Cheapest priced copy (Duel Decks: Elspeth vs. Kiora): $0.35 at Card Kingdom, near mint, their price list as of .
Sealock Monster 
Creature — Octopus · 5/5 · Uncommon · Duel Decks: Elspeth vs. Kiora · #42
This creature can't attack unless defending player controls an Island.
{5}{U}{U}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, target land becomes an Island in addition to its other types.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Elspeth vs. Kiora | 42 | Uncommon | $0.35 | — |
| The List | DDO-42 | Uncommon | $0.35 | — |
| Theros | 62 | Uncommon | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 15 September 2013The effect making the land an Island doesn't have a duration and will last indefinitely. The land retains any land types and abilities it already had. An Island has the ability “{T}: Add {U}.”
- 15 September 2013Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
- 15 September 2013An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
- 15 September 2013Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.