Some Lords go crazy after the sixty-million-year-long sleep, and get a bad case of sour grapes for being stuck in a machine for so long. They decide that they never really liked being alive anyway, and that everyone should join them in death as well. To that end, they modify their bodies to have a hoverboard base and an enhanced internal structure, much like the regular Destroyers.
Destroyer Lords are more machine-like than standard Necron Lords and are said to be the most maniacal of their kind, lacking any ability to empathize with other creatures due to their cold, calculating nature. Even mighty Necron Overlords find Destroyer Lords somewhat disconcerting, believing they have too willingly embraced the machine and may turn on their own kind when the galaxy is rid of organic life. As a result, many are outcasts and pariahs in Necron society.
In combat, it is said that the physical might of a Necron Destroyer Lord equals the mightiest Overlords. Whilst a Destroyer Lord dwarfs even that in terms of madness and all-out savagery, which would explain why other Necron leaders shun them. Most favor warscythes and voidblades.
Destroyer Lords serve no Overlord or Phaeron (and are a stand-alone HQ choice), because the less-crazy nobles are worried that the Destroyer Lords will turn on their fellow Necrons once all organic life is wiped out, and because they're afraid that Destroyer-ness may be contagious.
With the advent of Ninth edition the Destroyers Cult has been expanded to include different types of Destroyers, and the guys with hoverboards for legs that we've known and loved/hated for several editions are now a variant called Lokhust. With the classic Destroyer Lord being rebranded as the Lokust Lord. Oddly enough, despite Heavy Destroyers getting a brand new model, the Lokhust Lord and the regular Lokhust Destroyers are still left with greenrod syndrome as of the Necron 9th edition Codex release.