The Final Threat is a story that ran in the pages of Avengers Annual #7 and Marvel Two-In-One Annual #2. Written and drawn by Jim Starlin, known for his cosmic sagas at Marvel, it tells the story of a mad demigod, the heroes who arrayed themselves against him, and the fate of a tortured soul who strove to be human, but was in some ways much, much more. Starlin's work was so trippy that according to Sean Howe in Marvel Comics: The Untold Story, fans would send complimentary joints in with their fan mail!
Adam Warlock as a character evolved from a messiah- like figure to a cosmic wanderer, reflecting the angst of the times. His early stories recounted him battling his future self, The Magus, and the Universal Church of Truth, an intergalactic empire. At one point he acquired a soul gem, with the ability to steal the souls of others. As the story here begins, Warlock arrives on a far planetoid and finds a dying Gamora amongst the ruins. Gamora was a deadly assassin in the service of Thanos, and considered the most dangerous woman in the galaxy. She tells Warlock that Thanos plans to extinguish the stars in an act of stellar genocide. After taking her soul into his soul gem to relieve her suffering, he returns to Earth and enlists the Avengers and the Kree warrior Captain Marvel to help him stop Thanos.
Warlock then mysteriously disappears, and the Avengers learn of a massive space fleet approaching. They launch themselves into space to engage, and Iron Man and Thor tackle the ships while the rest of the Avengers blast their way into the Sanctuary II, Thanos' flagship. They fight their way through hordes of alien flunkies, only to discover that Warlock has arrived ahead of them. Warlock has discovered his friend Pip the troll aboard- but Pip is catatonic, his mind destroyed by Thanos. Along with Gamora, he has now lost the two people who meant the most to him.
Warlock has learned that Thanos is on the far side of the sun in another vessel, and that this was all a decoy. Warlock and captain Marvel fly there, leaving the avengers behind, and attack Thanos. Captain Marvel destroys the star gem apparatus that Thanos was constructing, but not the gem itself. Warlock meanwhile is killed in battle by Thanos. Thor and Iron Man arrive, having followed them, and Thor battles Thanos while Iron Man destroys the gem. As this is happening, Captain Marvel witnesses a strange tableau- another version of Warlock appears and takes the soul of the dead Warlock into the soul gem. We then see Warlock inside the world of the soul gem, a place of peace where he is reunited with Gamora and Pip, as well as the souls of all those he defeated in the past. They are no longer enemies however- everyone is at peace within the gem. His old enemies welcome him with open arms.
The story then segues into a dream that Spider Man, on earth, is having, where he "sees" the events happening in space- and sees further that the Avengers aboard Sanctuary II have been defeated after Thanos rallies his forces. He strips the soul gem from Warlock's brow and plans to use its power to blow up the sun. Turns out that the dream is actually a psychic distress call from Moondragon, one of the defeated Avengers. Spidey goes to the Fantastic Four for assistance, but only one member- Ben Grimm aka the Thing- is home, so together they blast off and land on Sanctuary II. They engage Thanos' forces but are defeated, however Spidey manages to escape and free the captured Avengers. It's everyone against Thanos at that point, but even that is not enough- until Spider Man manages to call forth Warlock from the soul gem.
Appearing as a being of fire and vengeance, Warlock is the one being Thanos cannot withstand, and he is defeated. The story ends with the Avengers and Captain Marvel mourning Warlock, even as he finds himself once more in the soul gem, at peace.
This was the original Thanos "event" story, way back when, and ends with his demise at the hands of Adam Warlock. None of the Thanos stories since have topped it, in my view, and yet they keep bringing the character back. Diminishing returns every time. For me the Thanos story ends with this one, and it's a good cosmic adventure.
This story has been collected in various reprint volumes over the years. My version is the recently released Epic Collection, which contains this story as well as a host of other issues, including several other classic stories. I recently purchased this from Amazon and found it to be a good buy.