Magus

Origin

The Magus was originally a future version of Adam Warlock created by the In-Betweener who came to believe that he was a god, and led his fanatical followers on a crusade to conquer the galaxy and eliminate all who were "impure". This impure consisted of  all non-humanoids and certain 'undesirables' such  as trolls. He uses a church called the Universal Church of Truth to attain his whims; this church treating him as a god.  He used his Black Knights to protect the Church and his Grand Inquisitors to seek out those who would oppose the Church and terminate them.

Adam Warlock learned of this and that the Magus was actually his darker self. He knew it was his own responsibility to stop this madness. He was trapped by the Church and they attempted to convert him to their faith but his will proved too strong. Warlock had the assistance of Gamora and Pip the Troll. He was also aided by an unlikely ally, Thanos. Warlock was able to travel his own timeline and find himself before he was to become the Magus. He absorbed himself into the Soul Gem and erased the Magus from existence, and all knowledge of him and the damage done by his Church were eliminated from the universe.

It is worth noting that the original incarnation of the Magus, created in the 1970's, sported a very appropriate for the time Afro although, while the character was created in the 70's, he is actually from the future, by which time Afros are more than likely no longer in vogue.

Infinity War

The second Magus is the manifestation of all the evil inside Adam Warlock. He was created when Adam Warlock cleansed himself of all good and evil while he obtained the Infinity Gauntlet. Warlock was unaware that this cleansing ever occured. The "good" cast off from his soul forms The Goddess. 

Magus gathered five Cosmic Cubes and used them to create evil doppelgängers of many superheroes and Thanos. The Doppelganger of Spider-Man would be revived after the Infinity War and plays a role in the storyline " Maximum Carnage".  He planned to replace the entire universe to how he saw fit. But the cosmic cubes were not enough for the Magus, he wished to obtain the Infinity Gems for himself. His plans were well concived. He used his power to put Eternity in a catatonic state. He also watched every major player in the universe closely and had every part of his plan go according to how he wished.

Magus also manages to collect the Infinity Gauntlet in addition, though when he first wields it, the gems are still forbidden to work in unison so he is almost defeated when his base is stormed by Kang and Dr. Doom.  Just as Magus is about to hand the gauntlet over to them, however, The Living Tribunal and Eternity allow the gems to work together again and Magus reasserts control.  He finally has the power he so long desired.

But the Magus was deceived. Aware that they were being watched, Thanos and Adam Warlock replaced the Reality Gem with a powerless facsimile. The Magus and his lust for power failed to notice this and thought he had full power and was now godlike. Adam Warlock battled him for control of the gems and knowing the truth about the gems proved to make him more powerful. Adam released Eternity and Infinity from himself. Magus was unaware that they are part of the same entity. Their combined might was  more than Magus could handle without a complete gauntlet. He was eventually defeated when he was sent into the Soul Gem.

Imprisoned in the Soul World

As a spiritual embodiment of a mere fraction of Adam Warlock's whole soul, Magus finds he has no substance in Soul World and none of its residents know he is even there.  He realizes this when, upon first arriving in soul world he spots two former Universal Church of Truth employees of his who had been absorbed into the Soul Gem years ago, Captain Autolycus and Judge Kray-Tor and Magus tries to talk them into being the first recruits for his new army, only to have them walk right through him and keep on going, never even having known he was there at all. 

When the Goddess starts the Infinity Crusade, Warlock is able to find out her true intentions and travels to the Soul World in an attempt to get the Magus' assistance. Warlock offers Magus the opportunity to merge with him and help him battle the Goddess. But, as much as the Magus despises his imprisonment, he hates Adam Warlock more than anything and would rather the universe be destroyed than to help him.  Magus attacks Adam Warlock in an attempt to return back to reality in Adam Warlock's waiting body. But he is soundly defeated by Warlock, who claims that Magus had no chance of winning. Warlock leaves Magus behind in the Sol World.

Magus sinks into despair until the end of the Infinity Crusade, when the Goddess is also absorbed into the Soul Gem and winds up in the same situation as Magus.  Magus antagonizes Goddess until she takes a swing at him, only to have her fist pass through him.  While they can see and speak to each other, they are still just ghosts, even in Soul World.

Captain Marvel

The Magus later reemerges in the pages of Captain Marvel Volume 4 as something of an energy vampire.  He torments Genis Vell and some of his old friends from Captain Marvel volume 3 in his non-corporeal form and drains the life force of hundreds of citizens of the planet he's on; nearly enough to give himself a new proper body.  Magus is stopped by Genis, but not before he manages to take mental control over Genis' ally, Moondragon (though this plot thread does not get followed up on).

Magus appears once more to antagonize Genis again, this time with the aide of Genis' own son, pulled out of time from the future and corrupted.  Genis only manages to defeat his son by willing himself to kill him while he is still an infant; and, while this has not yet occured in Genis' timeline, he apparently would truly follow through with it as the decision wipes his son's older self out of existence.
 

The New Magus (*Spoilers*)

When the fabric of space had become damaged, opening fissures and collapsing reality, Adam Warlock and the Guardians of the Galaxy try what they can to stop it. In addition to this, the Inhumans have detonated a device known as the T-Bomb, which created a hole in space known as the Fault, which threatened to consume the entire universe. 

 The New Magus

Receiving a warning from his teammates in the future, Adam Warlock did what needed to be done to stop the universes' destruction. He was able to stop the Fault from spreading, but this left him weakened. In order for this to work, Adam overlapped timestreams, making the sacrifice to save the universe. This allowed his darker side to take over, and he became the Magus.
 
Some of the Guardians were sent back in time to stop this by Kang the Conquered, who warned that all possible futures are becoming Magus futures. He gives them a Cosmic Cube to assist them. Starlord attempts to talk Adam into controlling his dark side, but fails. After a battle with the Magus and the apparent deaths of Mantis and Cosmo, Starlord is able to revert Adam temporarily with the Cube. But the Magus cannot be stopped, and Starlord ends Adam's life at his request.