Immortality, as Mr. Gold says, doesn't mean one can't be killed, but means to live forever. In a place with magic or not, they can always be killed with some certain weapons or items. ("Second Star to the Right")
However, living extremely long does not mean that the person has the ability of immortality. Fairies, Regina Mills, Zelena, Fendrake the Healer, Gothel, Seraphina, Pinocchio or Jiminy Cricket have lived a very long life, but this is due to their creation and nature, or using some magic to keep themselves young. Inhabitants of Neverland and the Land of Untold Stories also do not age as long as they are in those worlds. This is because time always stands still in there. Inhabitants of Storybrooke before Emma's arrival also do not age due to the time being frozen and no one changing.
How to Gain Immortality
- Holy Grail - Immortality is one of the magical abilities that the Holy Grail grants to the person who drinks from it. After Nimue corrupts herself and her Holy Grail magic, and becomes the first Dark One, this ability still remains in her and all the other Dark Ones. ("Nimue")
- Divine Magic - Deities have this ability because of their nature. It can also presumably be said that Deities are the creators of the ability of immortality since they are the oldest immortal beings. ("Last Rites")
- There are also immortal people who have magic like Maleficent, or creatures like Medusa. However, how they got this gift or why they have it is unknown.
How to Kill an Immortal Being
- Excalibur - While Holy Grail grants someone immortality and eternal life; Excalibur, whose original form is Holy Grail, was forged to cut immortal ties. It has the ability of killing both Merlin or any Dark One, as well as swipping away their immense magical powers. ("Nimue", "Swan Song")
- Dark One Dagger - Because the dagger is originally part of Excalibur, it also has the ability to kill any Dark One. It is the main and most-known item that can kill any Dark One, and that's why, it is said that the dagger is Dark Ones' weakness numerous times. ("Desperate Souls", "The Return" et al., "Nimue", "Swan Song")
- Dreamshade - When Hook stabbed Mr. Gold with the deadly poison dreamshade in the Land Without Magic, Mr. Gold had no option but to die unless he somehow exchanges someone else's life for his. But it is not known if it has the same deadly effect in a location with magic, since Hook poisoned Mr. Gold in New York City. ("The Queen Is Dead", "The Miller's Daughter")
- Another Immortal Being - It's possible for an immortal being to kill another immortal person who is gifted with the Holy Grail. ("Nimue")
- Soul Ripping Arrows - Immortal or not, there is no escape from the "Worse Place". Soul ripping arrows can also be used as an alternate way. ("Her Handsome Hero")
- Olympian Crystal - Originally belonged to the ruler of Mount Olympus, it has the ability of erasing someone's existence, even a powerful being like a god. ("Last Rites")
- Golden Flowers - According to the Evil Queen from the Wish Realm, golden flowers have the ability to kill an immortal being. ("Eloise Gardener")
- Author Powers - Though the Author's powers cannot destroy the Darkness itself, it can be used to destroy the host's magical powers and immortality. ("Leaving Storybrooke")
It is possible to hurt an immortal being with a simple sword and make them bleed. Also when Robin Hood shoots an arrow to Rumplestiltskin, he simply removes the arrow from his chest and continues to live like nothing ever happened. But they can still experience pain from being strangled by telekinesis. However, none of the situations can kill them. ("A Land Without Magic", "Lacey", "Swan Song")