A knife of unknown origin, the Demon-Killing Knife/Ruby's Knife was first seen with Ruby in "The Magnificent Seven." The Winchester brothers gained control of the knife and are in current possession of it. Meg and Castiel have also been known to make frequent use of this blade. Since its introduction, the knife has become one of the Winchesters' most common means of killing demons.
The demon Alastair survived being stabbed by the knife, but implies that if he was stabbed directly through the heart with it, he would die. Seconds later, Castiel telekinetically moved the knife down towards his heart, but all this did was cause Alastair more pain him, suggesting he was truly immune.
Alastair is the only demon to date to be shown immune to the blade, although it is unknown if Azazel or Lilith would be immune too, as they both seem to be more powerful than him. However, Lilith is shown to be frightened of the knife when Sam threatens her with it, so it is possible the knife could kill or at least harm her. However, as Alastair is her subordinate and that the knife's origin is probably unknown to demons (as Alastair was unfamiliar with the design) it most likely cannot harm her but she wouldn't risk it as it would mean she would be unable to break the final seal.
At first glance, the knife appears to be an ordinary hunting knife with a curved, serrated-edged blade and a wooden handle. However, upon closer inspection, symbols can be seen engraved along the blade.