While I agree that Nightblood is an extremely cool sword, please permit me to respectfully disagree with your assessment of the coolest sword ever.
In my (not so) humble opinion that honor goes to the twin brother swords of the Elric of Melnibone series by Michael Moorcock.
from
http://www.stormbringer.net/elric.htmlThe Rune blades: Brought to earth centuries before and wielded by the Melnibone emperors, these blades where turned over to the Gods and placed in the pulsing cavern when they became too powerful to control. They are known by the names StormBringer and MournBlade. They have the ability to give the wielder unparalleled strength. They are also sentient and require feeding to appease them. This feeding is in the form of souls taken during battle. Many of Elric's enemies plead not to be killed by his fearsome blade, StormBringer, as their souls are sent straight to hell for eternity. Although Elric is able to overcome his genetic weaknesses, and for the first time in his life he no longer needs to ingest his sorceress potions to sustain his life, this strength is gained at the price of a symbiotic relationship with his blade. Many is the time he will regret unsheathing it, as StormBringer has a taste for friend and foe alike.