Board Thread:Show-Related Questions and Answers/@comment-26116863-20151117011253/@comment-25926288-20151117011820

Arthur cut him with Excalibur before Hook knocked him down in the standoof in the woods.

Basically, Hook could not be heal because not magic could not heal such a wound, only ones immortality could subdue it. So, Emma mde Hook immortal so that he could survive that cut.

In simple terms, Merlin or the DO could get cut with the dagger or Excalibur and would have to heal naturally. Normal people just will die.