Forget all the jabber about Ultron, the true arch enemy of the Avengers is Kang the Conqueror. Ultron is merciless and relentless true, but Kang is all that and much much more. Let me explain.
The story of Kang begins in Fantastic Four #19 when the Fab 4 seek out Doctor Doom's time travel machine ensconced in his New York state castle and use it to go to ancient Egypt to seek a cure for Alicia's blindness. They find Rama Tut, the ruthless ruler of the area, who it turns out knew them quite well. He was a time traveler from Earth's future, a possible descendant of Doc Doom himself who was bored by the lax life of 3000 AD and sought adventure in the past. He found that adventure and more and came to rule the Egyptians. Eventually the FF find a way to defeat Rama Tut who escapes in his time sphere as they themselves return to their own time, having failed in their mission but hopeful.
Rama Tut turns up again in Fantastic Four Annual #2 when he rescues his presumed ancestor Doc Doom who had been floating in space for a bit.
He shares a moment with is counterpart before the two part ways to pursue their individual dreams of conquest. The pair even speculate that they might be the same man from two different times.
We find out in The Avengers #8 what became of Rama Tut when the Assemblers confront a mysterious and dangerous occupant of a mysterious UFO.
That man is Kang the Conqueror who hails from the distant future of 4000 AD and who after having conquered all that he could survey came to the 20th Century to confront a culture which might prove more of a challenge. He finds that challenge when the Avengers attack, but swiftly they are captured and imprisoned on Kang's ship. Thanks to the ingenuity of Rick Jones and his Teen Brigade who pretend to serve Kang, the Avengers are freed and defeat the Conqueror in his first foray.
Weirdly the very next issue of The Avengers sees the team battle the enigmatic Immortus, who partners with the Masters of Evil to try and defeat the Assemblers.
His schemes fail despite his pitting such mythical entities as Paul Bunyan, Merlin, and even Hercules (not that one) against them. Of course we all know now that Immortus will be revealed to be Kang in a later period of his time-traveling existence.
Kang himself returns in the very next issue of The Avengers where he uses a robot double of Spider-Man to lure the Avengers into a trap in a booby-trapped temple in Mexico.
Kang, who at this time still might be a future incarnation of Doctor Doom seems to have Doom's proficiency with robots. Thanks to the timely intervention of Spidey himself, the Avengers survive Kang's attack, but the die seems to be cast that he has taken a supreme interest in the Assemblers.
More to come.