2018-07-13 : Casting an exiled card causes it to leave exile. You can't cast it multiple times.

2018-07-13 : Dark-Dweller Oracle doesn't change when you can cast the exiled card. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty. If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining.

2018-07-13 : If you don't cast the exiled card, it remains in exile.

2018-07-13 : You can sacrifice Dark-Dweller Oracle to pay the cost for its own ability.