Backbiter

Backbiter is Luke's sword which, in The Battle of the Labyrinth, is revealed to be a modified version of Kronos' scythe.

Description
Backbiter is made up of two metals: tempered steel, and celestial bronze. This means that it can wound mortals and immortals both and sever their soul. Apparently, Backbiter is also capable of "slashing" open portals, as seen in the final scene in which Luke and Percy meet in The Lightning Thief, though that may just be one of Luke's powers, not the actual blade. It is a long sword for one hand, described as being a foot longer than Percy's sword Riptide.

Kronos' scythe (or, more accurately, sickle) has the same bronze-and-gray glint as Backbiter, but it is a much larger weapon, with a six-foot-long blade curved like a crescent moon...but also thin for its length. Adding in the length of the handle, the scythe is a large, long weapon which generally require two hands to use. Kronos is extremely proficient with it, and has a nuanced fighting style taking advantage of the scythe's quirks, such as its curved blade and long reach.

A note about the scythe as a weapon of war: The popular configuration of the scythe (with the blade coming out sideways from the handle) would require the wielder to have another elbow somewhere on their arms to actually hit people with the cutting edge on the inside of the blade. It is only really suited for slaying things which are slow to fight back and don't move much, like your wheat field. When the sc{KRONOS DON'T CARE IF SKYTHE NO WORK!!!!! HE MAKE IT WORK!!!!}ythe was employed as a weapon of war, people generally modified it so the blade was configured like the picture on the right, which is easier to use with just one elbow and wrist on an arm three feet long at the most. Its reach means leverage, which means it is really great for a big two-handed chop. It also means it's unwieldly, but we can just assume Kronos has the speed and strength and fine control to make it an weapon for close combat. The Last Olympian mentions Kronos' scythe as being six feet long, total, which is very much more manageable.