In the time after the Third War, as the Forsaken were establishing themselves as members of the Horde, a powerful soldier named Constance Corvasius began to make a name for herself. She was a fierce corpse who saw through a single enchanted eye attached to her tongue and spoke in slow hissing noises. Corvasius soon rose to the rank of Executor and was given a unit to command, which she named the Scytheguard and later the Scythe of Sylvanas. It's rumoured that the name was inspired by an account she had heard of a failed expedition, in which Forsaken soldiers strapped sickles and other farming equipment to themselves to replace limbs they'd lost in battle.
The Scythe proved themselves to be an effective piece of the Forsaken war machine and served in Horde campaigns both in Outlands and in Northrend. However, after the death of the Lich King, Executor Corvasius disappeared into Silverpine Forest with a mind to hunt the Worgen and defend the Dark Lady's lands. When she failed to report in, her unit was disbanded and the remaining members of the Scythe were stationed elsewhere.
Shortly before Deathwing emerged and tore Azeroth asunder, a group of Forsaken - both those who had fought under Corvasius and those who had just heard her story - banded together again under the name and crest of their former commander.
Now the Scythe of Sylvanas is a brotherhood of Undeath. A curious combination of all manner of Forsaken: members of the Royal Apothecary Society, followers of the Cult of the Forgotten Shadow, Deathstalkers, Scourge-freed, Val'kyr risen, and more. They devoted themselves to protecting the interests of their race and took pride in the mobility afforded them by their mercenary ways.
The Scythe involves itself in helping to meet the day-to-day needs of the Undercity, as well as helping to protect their land and people. It's not always glorious, but it is necessary, and it's usually bloody enough to satisfy. On any given day, the Scythe might find itself gathering supplies to replace those sent to aid the horde forces, patrolling the borders and filling in for the soldiers who are now far from their posts, tracking down rare reagents and test subjects for the Royal Apothecary Society, or attempting to rescue a Forsaken brother or sister who was unlucky enough to be captured by the Alliance.