Roil Elemental

Flying
Landfall — Whenever a land you control enters, you may gain control of target creature for as long as you control Roil Elemental.

Secret Lair Drop

A vortex that devours everything—even the souls of the living.

Zendikar

Flying
Landfall — Whenever a land enters the battlefield under your control, you may gain control of target creature for as long as you control Roil Elemental.

If Roil Elemental ceases to be under your control before its ability resolves, you won't gain control of the targeted creature at all.

If Roil Elemental leaves the battlefield, you no longer control it, so all of its control-change effects end.

The landfall ability triggers whenever a land enters under your control for any reason. It triggers whenever you play a land, as well as whenever a spell or ability (such as Rampant Growth) causes you to put a land onto the battlefield under your control. It will even trigger when a spell or ability causes another player to put a land onto the battlefield under your control (as can happen with Yavimaya Dryad's ability, for example).

When a land enters under your control, 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 that resolves.

You 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).