The Dark One Dagger, or the Dark One's Dagger, is a magical item featured on ABC's Once Upon a Time. It first appears in the eighth episode of the first season. The dagger's main use is to control the Dark One. It is majorly used to stab the Dark One with it, and the power of the Darkness passes to the one who kills them. After then, the new host's full name appears on the blade. ("Desperate Souls, "Operation Mongoose Part 2" et al.)
The significance of the Dark One's name on the dagger is an allusion to the original Rumpelstiltskin fairytale, where Rumplestiltskin made a deal with the miller's daughter to discover his name.