Carnwennan

Carnwennan, or Carnwenhau (meaning "little white hilt") is a dagger wielded by King Arthur in some versions of Arthurian legend, exclusively in Welsh traditions.

Abilities:
Carnwennan is associated with the ability to cloak its user in shadows. This may be meant to imply that the holder can use it to turn invisible, or it may literally imply that they can cover themself in darkness.

The dagger also seems to carry immense cutting power, as Arthur is described as using it to slice an opponent in half. This may simply imply that it carries so much power that striking an opponent with it would tear them in half, or it may imply that the dagger's blade has a longer functional reach than its physical form, making it possible to slice someone in half with it.

Culhwch and Olwen:
In Culhwch and Olwen, Arthur names it as one of the few things in the world which he will not give to Culhwch, marking its importance to him. Later in the same story, he uses it to kill the witch Orddu by slicing her in half.

Welsh Triads:
In the Welsh Triads, Carnwennan is listed as one of the three holy weapons given to Arthur by God, the other two being Excalibur and Rhongomiant.

Le Morte d'Arthur:
In Malory's telling of Arthurian legends, Arthur wields a dagger that he uses to kill a giant. It can be inferred this is Carnwennan, however it is not named. The use of it to kill a giant may also be a reference to Cuhlwch and Olwen.