Roil Elemental

• WWizards of the Coast2009-10-01
  Official Ruling
  If Roil Elemental leaves the battlefield, you no longer control it, so all of its control-change effects end.
• WWizards of the Coast2009-10-01
  Official Ruling
  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).
• WWizards of the Coast2009-10-01
  Official Ruling
  If Roil Elemental ceases to be under your control before its ability resolves, you won't gain control of the targeted creature at all.
• WWizards of the Coast2024-11-08
  Official Ruling
  A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.
• WWizards of the Coast2024-11-08
  Official Ruling
  Whenever 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.).
• WWizards of the Coast2024-11-08
  Official Ruling
  A 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.

Rulings via Scryfall