The Reapers are a roving band of living weapons who want to give peace a chance — by killing everybody. They're led by a Deathbringer. One is also named 'Klaxa'. The names (and even some species) of the others are unknown.
IDW Comics continuity
According to the Deathbringer, the Reapers were warriors who caused utter destruction. However, their prowess caused them to be feared even among their own, leading to isolation and bringing them towards madness. They found each other and resolved to use their powers to destroy war within the universe by eliminating violent races and removing things worth fighting over. They laid waste to the strategically important planet, Mumu-Obscura, to remove it from the equation and spare needless future conflict. When Decepticon Sixshot arrived on Mumu-Obscura to locate his 'friends', the Terrorcons, the Reapers attacked him as a test of his abilities. The Deathbringer offered Sixshot a place among them if he would kill the Terrorcons, but Sixshot was still unable to sever his connections to the Decepticons. The Reapers accepted his decision and said that they would cross paths with him again, hoping that he would later be able to accept their offer.
After Sixshot was summoned to Earth, the Reapers followed, vowing to destroy whatever was left after Sixshot's work was done. After a period of observation in Earth's orbit, they decided that Earth should be destroyed and did not care whether Sixshot helped them or fought them. However, this was observed by Nemesis Prime, who had his own agenda for Earth, and sent Galvatron to stop the Reapers. The Reapers attacked the Decepticon base, prompting Megatron to recall Sixshot from his battle with the Autobots. When Sixshot arrived, they again requested he join them or be destroyed. When Sixshot's rival Starscream joined the battle, Sixshot decided to join with the Reapers, but Starscream quickly deactivated him with a shutdown command. Starscream then proceeded to bomb the Reapers and the Decepticons soon joined the attack. During the battle with the Decepticons, the Reapers initially had the advantage and were able to kill the Battlechargers. Galvatron covertly turned the Deathbringer into a harbinger of unlife, sending him to destroy the others simply by touch. Galvatron also blasted a bird-like creature who tried to attack him. The Deathbringer decimated his brothers, giving the Decepticons the edge they needed to finish off the remaining Reapers.
Klaxa was the only Reaper identified by name. He was a dragon like alien capable of breathing fire. Klaxa was among the Reapers who were present to test Sixshot's abilities on Mumu-Obscura, breathing fire hot enough to threaten Sixshot's body armor. After Sixshot declined their offer to join, he departed with his brothers. When the Reapers attacked the Decepticon base, Klaxa was part of the first wave of Reapers that attacked. Once the Reapers realized that Deathbringer was destroying the others by touching them, Klaxa tried to incinerate his former leader, but failed, and soon succumbed to his touch of death.
This Ravenous was among the Reapers that tested Sixshot, and was the only one besides the Deathbringer that Sixshot was able to identify. Ravenouses are infected with Cy-Viruses that strip all restraint away from them, making them ferocious in battle. During the attack on the Decepticon base, the Ravenous spotted Galvatron after he infected Deathbringer, and tried to destroy him. However, Galvatron destroyed him with but a single shot of his cannon.
This hooded Reaper was among the group that tested Sixshot. His face seemed to be composed of energy, and he was capable of channel energy at his opponents. He was among the group that attacked the Decepticon base in Oregon. After realizing that Deathbringer was destroying their brothers, the hooded Reaper ordered Klaxa to incinerate him, but Klaxa was unsuccessful. The Reaper presumably perished shortly afterwards, whether at the hands of the Deathbringer or the Decepticons is unknown.
This Reaper only was seen once during the attack on the Decepticon base in Oregon. It appeared to be some kind of mechanoid with the ability to spin around rapidly like a whirlwind. This Reaper decapitated Runamuck using this power. It presumably died with the rest of its brethren, either by Decepticon hands or the corrupted Deathbringer.
This Reaper was part of the initial group that tested Sixshot. It appeared to be of mechanoid origin, with an upper body set on tank treads and a long cannon situated in its chest. Sixshot surmised that it was manufactured by its creators for the explicit purpose of mass destruction. Later, when the Reapers attacked the Decepticon base in Oregon, the Tank Reaper conversed with the Deathbringer over what to do about Sixshot. Finally, this Reaper was killed by Blitzwing, who blew it up with a tank mode cannon shot.
This Reaper was part of the group that tested Sixshot on Mumu-Obscura. It was able to project energy of a density that resisted even Sixshot's sonic blasters, as well as teleported the Terrorcons on the field after they were done testing Sixshot. During the battle in Oregon, the Invertebrate Reaper unleashed its energy on Runabout, causing him to explode. Its corpse was later seen at the feet of the victorious Decepticons, lying next to the Deathbringer, leaving its killer unknown.