Cheapest priced copy (Zendikar): $15.99 at Card Kingdom, near mint, their price list as of .
Roil Elemental 
Creature — Elemental · 3/2 · Rare · Zendikar · #62
Flying
Landfall — Whenever a land you control enters, you may gain control of target creature for as long as you control this creature.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $15.99 to $34.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Secret Lair Drop | 155 | Rare | $22.99Out of stock | $34.99 |
| Zendikar | 62 | Rare | $15.99 | $22.99Out of stock |
6 rulings from Wizards of the Coast
- 1 October 2009If Roil Elemental leaves the battlefield, you no longer control it, so all of its control-change effects end.
- 1 October 2009You may target a creature you already control with Roil Elemental's ability. This will usually have no visible effect, but it will overwrite any previous control-change effects. For example, if you gain control of Roil Elemental until the end of the turn (with Mark of Mutiny, perhaps), then its landfall ability triggers and you target Roil Elemental itself, you'll gain control of it indefinitely (since its control-change effect will last for as long as you control it).
- 1 October 2009If Roil Elemental ceases to be under your control before its ability resolves, you won't gain control of the targeted creature at all.
- 8 November 2024A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.
- 8 November 2024Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).
- 8 November 2024A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.