The coolest sword ever in my opinion. Why? well there are many reasons but I'll put the ones I think are the best.
It's very ancient, thousands of years old, and was made by Vorador.
It's a sword that goes to many phases, for it began as a a vampiric blade that has the ability to suck the blood of it's victims.
It's sentient for it contains the soul of (spoiler alert)Raziel.
It's also the one object capable of disrupting the flow of time and alter it, which is something that occurs several times.
And at the end it becomes Kain's most powerful weapon, and the last hope for Nosgoth(that's the name of the world inside the games.
BTW this is the statue from where Kain first found the Reaver at Avernus Cathedral.