Rune claws are a two handed melee weapon. They are the second strongest pair of claws. To wield them, a player must have at least 40 Attack.
Rune claws can be obtained either by using the Smithing skill to make a pair, or looting Ninja implings for them. The requirements for making them, is completing the Death Plateau quest, along with 98 Smithing. To make them use a rune bar (with another in the inventory) on an anvil, with a hammer in the inventory. This will grant the player 150 Smithing experience.
A player wielding rune claws
A player using the rune claws' special attack, Impale
Rune claws have a special attack, Impale, which consumes 25% of the player's special attack energy, and deals 10% increased damage. The special attack increases the time until the next attack by one tick (0.6 seconds), and delays the actual special attack itself by the same amount of time.
They are one of only two rune metal weapons to have a special attack, the other being the rune throwing axe.
For an exhaustive list of all known sources for this item, see here