Goblin Dark-Dwellers — ruling 1

You cast the card during the resolution of the enters-the-battlefield ability, not later in the turn. If it's a sorcery card, ignore the timing restrictions based on it being a sorcery card. Other timing restrictions, such as "Cast [this card] only during combat," must be followed.

Goblin Dark-Dwellers — ruling 2

If you can't cast the target instant or sorcery card, perhaps because there are no legal targets available, or if you choose not to cast it, it will remain in your graveyard.

Goblin Dark-Dwellers — ruling 3

If you cast a card "without paying its mana cost," you can't pay alternative costs such as surge costs. You can pay additional costs. If the card has mandatory additional costs, you must pay those.

Goblin Dark-Dwellers — ruling 4

If the instant or sorcery card has {X} in its mana cost, you must choose 0 as its value.

Goblin Dark-Dwellers — ruling 5

If an instant or sorcery card you cast this way is countered, it will be exiled.

Goblin Dark-Dwellers — ruling 6

If an instant or sorcery card you cast this way goes to a zone other than exile or a graveyard, perhaps because one of its abilities says to put it into its owner's hand, it won't be exiled. This is true even if the card would be put into a graveyard later that turn.

Goblin Dark-Dwellers : règles, fonctionnement et interactions (MTG)

Goblin Dark-Dwellers is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Menace (This creature can't be blocked except by two or more creatures.) When this creature enters, you may cast target instant or sorcery card with mana value 3 or less from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead.

How it works : Goblin Dark-Dwellers

Original English text — Wizards of the Coast / Scryfall

#1January 22, 2016· Wizards of the Coast
You cast the card during the resolution of the enters-the-battlefield ability, not later in the turn. If it's a sorcery card, ignore the timing restrictions based on it being a sorcery card. Other timing restrictions, such as "Cast [this card] only during combat," must be followed.
No other card is named in this ruling.
#2January 22, 2016· Wizards of the Coast
If you can't cast the target instant or sorcery card, perhaps because there are no legal targets available, or if you choose not to cast it, it will remain in your graveyard.
No other card is named in this ruling.
#3January 22, 2016· Wizards of the Coast
If you cast a card "without paying its mana cost," you can't pay alternative costs such as surge costs. You can pay additional costs. If the card has mandatory additional costs, you must pay those.
No other card is named in this ruling.
#4January 22, 2016· Wizards of the Coast
If the instant or sorcery card has in its mana cost, you must choose 0 as its value.
No other card is named in this ruling.
#5January 22, 2016· Wizards of the Coast
If an instant or sorcery card you cast this way is countered, it will be exiled.
No other card is named in this ruling.
#6January 22, 2016· Wizards of the Coast
If an instant or sorcery card you cast this way goes to a zone other than exile or a graveyard, perhaps because one of its abilities says to put it into its owner's hand, it won't be exiled. This is true even if the card would be put into a graveyard later that turn.
No other card is named in this ruling.