Origins

 The Scythe was made by a group of women known as the Guardians. Its purpose is to be used by the Vampire Slayer to kill the last Old Ones, which are original and pure demons.   The Guardians magically put the Scythe in a rock, and i  t could only be removed by a rightful Slayer in a time of dyer need. The Scythe was then kept secret and hidden from the Shadow Men, who would later be known as the Watchers Council.   When a Slayer is wielding the Scythe they can sense the natural power within, since it possesses the "Essence of the Slayer". 
 
"It's old... it's strong and it feels like...like it's mine" - Faith  
 
The Scythe's proper name is mʔ. The "ʔ" symbol represents glottal stop. This is significant due to the fact that glottal stop is a phonetic plosive that is voiceless and is produced by completely obstructing the airflow in the vocal tract.  
  
Please note that The Scythe is more similar to a Lochaber Axe than a traditional Scythe. Along with the blade, the Scythe has a wooden stake on the end of the handle which makes it an ideal vampire killing weapon. (Beheading and Staking are the more efficient ways of killing a vampire.) 

2003: Enter Buffy the Vampire Slayer

 

As the group were researching ways to stop The First. Spike found an inscription that lead him to a cave wall. He tells Buffy of his findings and she found the Scythe, underground while battling Caleb. She later uses it to kill Caleb and to fight the First Evil. Each Slayer that holds the Scythe says that it feels like ancient power - gentle yet strong.  A lternatively, powerful magic users like WIllow do not sense any power and she says "It must be a Slayer thing."  Willow later uses the power of the Scythe to activate all potential slayers around the world.