The Lethal Legion was first formed by the Grim Reaper to destroy the Avengers. The other members included Man-Ape, Swordsman, Power Man and the Living Laser. Man-Ape defeated Black Panther in combat and kidnapped his fiancee Monica Lynne. He took them to a secret chamber where he was held captive and met the rest of the Lethal Legion. Grim Reaper gathered his associates because they all had a deep hatred for the Avengers and had plans to eliminate them once and for all. The Grim Reaper sent his Lethal Legion to the water main that ran beneath the Avenger's Mansion and a nearby power station to ambush the Avengers. Black Panther escaped from his shackles and warned the Avengers of a possible trap but is eventually taken down in combat by the Grim Reaper. Goliath and the Scarlet Witch would come into conflict with the Swordsman and Power Man at the water main but they are defeated. Captain America and Quicksilver come into conflict with Man-Ape and the Living Laser at the power station but they also fall in combat as well. The fallen Avengers are held captive inside a giant hourglass that begin to fill up with deadly vapors. Power Man appears with a fallen Vision and places him inside the hourglass and hands the Grim Reaper personal files on the Avengers. Grim Reaper reads the file on the Vision and discovered that his brother's brain patterns was preserved to become the mental basis for the Vision. The Grim Reaper shatters the hourglass with his scythe to save his brother but they realize that the Vision inside was really Power Man and that the android had traded places with his enemy. The Avengers are free and they defeat all the members of the Lethal Legion.
The Lethal Legion was created by Roy Thomas and Sal Buscema in 1970 and first appeared in The Avengers # 78.
The Grim Reaper would reform a group against the West Coast Avengers. It was also reformed by Count Nefaria to drain the members powers for his own. Then later Satanish would form his own against West Coast Avengers that would lead to Mockingbird's apparent death.
The Grim Reaper would form a new group of criminals, that also included his brother Wonder Man to take down Norman Osborn. They were stopped by the Dark Avengers.
The Lethal Legion reappear with Absorbing Man, Mr. Hyde, Tiger Shark and Grey Gargoyle as their members. They would come into conflict with the Avengers after they take several bags of cash from an armor truck and try to escape on a boat off the New York Harbor. Hawkeye and several of his teammates realize that his archery skills are a bit off when he actually misses his target with an arrow.
The Lethal Legion of Earth-1298 (Mutant X) consists of; Brother Voodoo
, Fin Fang Foom
, Devil Dinosaur
, Moon Boy
and Sherry the Showgirl