A Deathbringer is a Series Two sentient mechanoid designed to visit planets wracked by plague and terminate the dying painlessly (if they so choose to die then, of course).
Just who made the Deathbringers is unknown, but they maybe should have chosen a nicer-sounding name for them.
Marvel Comics continuity
Siren read an ancient Cybertronian legend about the "Deathbringer", which eerily prophesied the Deathbringer's encounter with the Matrix and subsequent rampage.
A dying Deathbringer mechanoid came into contact with the Matrix, which had been accidentally ejected into deep space along with Optimus Prime's body. (It was later revealed that the Deathbringer encountered the Matrix on the moon VsQs, where Optimus' body had landed.) The Matrix, curious for fresh experiences, re-created it. But in the process, its programming became warped, and where once it offered death as a respite, now it spread death indiscriminately...
The re-created Deathbringer found its way to Earth, where it rained destruction down upon a human city. Optimus Prime waffled about whether or not they could destroy their only clue to the location of the Matrix, eventually throwing a rock at it. (It was a big rock!)
Nightbeat, meanwhile, correctly identified the Deathbringer's origins and convinced the Deathbringer that, since the Matrix's power was eating away at it, it should destroy itself.
Matrix-empowered bad guys: 0
IDW comics continuity
A Deathbringer commanded the Reapers, a group of robot "bogeymen" who seek to end all war in the Universe by eliminating everything worth warring over. This Deathbringer extended an invitation to Sixshot to join their group, though Sixshot declined as he could not sever all sentimental attachments to the Decepticons. The Deathbringer took the rejection in stride, declaring that Sixshot would cross paths with the Reapers again some day, and perhaps then, he would be ready to join.
When Sixshot was called to Earth, the Reapers followed and launched an assault on the Decepticon base. Deathbringer again made his pitch to Sixshot, helped by Starscream deliberately baiting the Decepticon, but the instant Sixshot signed on for the Reapers, Starscream deactivated him. Following that, Deathbringer was attacked by Galvatron and infected with death itself, turning him into a shuffling horror that spread death and decay among the other Reapers by touch before he finally collapsed. The Deathbringer, his brethren, and their dream of ending war all became casualties of Nemesis Prime's ambitions.
- The events of the Marvel story are shown only in flashback in the U.S. comic. Ratchet's presence places it shortly before Megatron's return in U.S. #56.
- The Marvel continuity's version of the Deathbringers (in the original, pre-Matrix-affected form) bear a striking resemblance to the Cylon Centurions from the original Battlestar Galactica TV series.