Immortality

"Immortal means to live forever, it doesn't mean one can't be killed."

- Mr. Gold to Lacey

Immortality is a magical ability featured on ABC's Once Upon a Time. It first appears in the first episode of the first season.

Description
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.

But as Mr. Gold says, immortality doesn't mean one can't be killed, but means to live forever. It is proven when Hook stabbed Mr. Gold with the deadly poison Dreamshade and Mr. Gold had no option but to die unless he somehow exchanges his life for someone else.

The Dark One's Dagger is also the main and most-known item that can kill any current Dark One, and that's why, it is said that the dagger is Dark Ones' weakness numerous times. And because the dagger is originally part of Excalibur, the blade also has the ability to kill any Dark One.

It is possible to hurt a Dark One with a simple sword and make them bleed, but it cannot kill them. Also when Robin Hood shoots an arrow to Rumplestiltskin, he simply removes the arrow from his chest and continues to live like before.

Deities also have this ability, as they are the ones who gifted the Holy Grail to mankind in the first place. but much like the Dark Ones, Deities also have their own "weakness" called the Olympian Crystal, which can, not just kill, but erase their existance.