The Thanosi are the villainous faction of the Marvel comic miniseries Infinity Abyss by Jim Starlin.
They are a group of rogue hybrid clones created by the original Thanos.
After the events of Infinity Gauntlet, the original Thanos gave up on his nihilistic views and genocidal plans and created in his laboratory the Thanosi clones, combining his DNA with the genes of various heroes and villains, in order to test the abilities of possible allies in his Battle against the death god Walker.
Allegedly, Thanos sent four clones to fight Ka-Zar, Thor, Hulk and X-Man (Nate Grey) and help the Avengers fight the Rot entity during the Celestial Quest. Eventually Thanos decided that the Thanosi were a failure due to their overly destructive and nihilistic behaviour and closed them in a storage in his laboratory.
Later, the remaining, Armour, X, Warrior, Mystic and Omega were set free by Atlez, who planted into them the purpose of rescuing Adam Warlock from an insane asylum. But the five Thanosi reached an Agreement to find and kill Warlock in order to bring Oblivion to the universe. Omega went to feed upon planets, while the Others decided to kill the original Thanos, and Armour estabilished a nihilist sect on the planet Baligiest.
They were eventually defeated and destroyed by the combined efforts of Thanos, Adam Warlock, Doctor Strange, Gamora, Spider-man, Moondragon, Captain Marvel (Genis-Vell) and Pip the Troll.
Armour: Created with the abilities of Iron Man, he was first seen speaking to the followers of a nihilistic cult on planet Baligiest, where he spotted Gamora among the crowd and ordered the cultists to kill her but she managed to escape. He was probably the least powerful of the Thanosi and his outfit looks like Iron Man's armor. He is destroyed by Thanos after calling him father.
Mystic: The hybrid of Thanos and Doctor Strange. He had a certain amount of mystical talent, but he was no match for the real Doctor Strange as he managed to imprison Mystic into another dimension.
Omega: The hybrid of Thanos and Galactus, and the leader of the Thanosi. He is allegedly more powerful than the real Galactus but his reflexes are rather slow. After consuming a few planets, and sensing the Death of his fellow Thanosi, Omega decided to do things by himself, as he traveled to Earth in attempt to kill Atleza. He was lured into a deserted planet by the heroes, as they destroyed his ship, and ultimately killed when Thanos destroyed the planet with a nuclear explosion of interplanetary magnitude.
Warrior: The Thanosi with the abilities of Gladiator of the Shi'ar Imperial Guard. His physical strength is four times bigger than Thanos, but Thanos considered him the least Dangerous of the Thanosi due to his dull wits and bad temper. Warrior was constantly restrained by X to avoid trouble. He was defeated by Adam Warlock when he used the Soul Gem to absorb his soul.
X: The Thanosi gifted with the mental powers of Charles Xavier, and Omega's second in command. He is the smallest of the Thanosi, with a short chubby body and a very large head. He disguised himself as the original Thanos and brainwashed Moondragon. Later Moondragon was free from X's mind control and got her Revenge with a mental illusion, leading X to commit suicide with a smile.
Unnamed Thanosi #1: The Thanosi who fought Ka-Zar. He had the ability to grow to giant size. His original purpose was to test how a regular human like Ka-Zar can survive against a powerful cosmic villain, in preparation for the fight against Walker. This Thanosi went rogue and planned to use the terraforming machine of the Savage Land to grow parasitic plants all over the galaxy, however this Thanosi was a low-level model, weaker than the original Thanos, and was defeated by Ka-Zar with the destruction of the terraforming machine. Sonetime later, this Thanosi tried to manipulate the Hulk into releasing him from the pocket dimension he was imprisoned but Hulk was saved by X-Man (Nate Grey) and the two teamed up to defeat the Thanosi.
Unnamed Thanosi #2: The Thanosi who fought Thor and teamed up with the monster Mangog. His original purpose was to prepare Thor for the fight with Walker and to study Mangog, as the monster is a being powered by a large amount of souls, just like Walker. This Thanosi and Mangog spread death and destruction on the planet Rigel-3 and then attacked the world known as Nucleus of Hope to capture Tarene, the chosen designate to guard the Chalice of Ruins and the Illumination Stone. The unnamed Thanosi was empowered by the Stone and the Chalice, but Thor eventually received by Firelord special pieces of armor blessed with the Odin Force and Mejingjord the Belt of Strenght, and was able to destroy the Illumination Stone and beat the Thanosi. Tarene wept for the victims of the Thanosi and her magical tears burned the Thanosi to a crisp.
Unnamed Thanosi #3: The Thanosi from the Celestial Quest story. He tried to kill Mantis and her son Quoi and become the all-father of his own pantheon of gods, created by mutating his minions. However, he was eventually forced to team up with the Avengers and Lady Death herself in order to destroy the Rot entity. After the entity was defeated, the Thanosi disappeared without a trace.
The Thanosi were made by Jim Starlin in order to retcon the appearances of Thanos in Ka-Zar, Thor, Hulk/X-Man Annual and Avengers: Celestial Quest, as he thought those stories did not follow Thanos character development at the end of "Infinity Gauntlet".
Thanosi was originally a fan-made name by the website Marvunapp, but it eventally became official.