Cheapest priced copy (Commander Legends): $1.49 at Card Kingdom, near mint, their price list as of .
Stormtide Leviathan 
Creature — Leviathan · 8/8 · Rare · Magic 2015 · #80
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
All lands are Islands in addition to their other types.
Creatures without flying or islandwalk can't attack.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $1.49 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends | 407 | Rare | $1.49 | — |
| Magic 2011 | 74 | Rare | $1.49 | $6.99 |
| Magic 2013 | 70 | Rare | $1.49 | $5.99 |
| Magic 2015 | 80 | Rare | $1.79 | $5.99 |
4 rulings from Wizards of the Coast
- 18 July 2014Stormtide Leviathan's second ability causes each land on the battlefield to have the land type Island. Each land thus has the ability "{T}: Add {U}." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary or basic.
- 18 July 2014Stormtide Leviathan's third ability affects all creatures with neither flying nor islandwalk, regardless of who controls them. They can't attack any player or planeswalker.
- 18 July 2014Islandwalk cares about lands with the land type Island, not necessarily lands named Island.
- 18 July 2014If Stormtide Leviathan loses its abilities, all lands on the battlefield (including those that enter the battlefield later on) will still be Islands in addition to their other types and will still be able to tap to produce {U}. The way continuous effects work, Stormtide Leviathan's type-changing ability is applied before the effect that removes that ability is applied.