...and this cover by John Buscema.
What wonderful additional villains did they bring to face Earth's Mightiest Heroes?
A Speedy Foe
Master of Sound
Black Panther and the Fantastic Four (returning again in Fantastic Four #56, this time as a man made of sound...except for his sonic blaster hand).
...and they had been charter members of the team, returning to face off against the Avengers again...as they would do in the future!
A Spy In The Ranks
Dane Whitman, who took over from the original Masters of Evil's original Black Knight...
...but he was really a hero, and ended up helping out the Avengers against the villains, and their mysterious leader....
The New Leader
Baron Zemo either....). Underneath the cowl of the Crimson Cowl was Jarvis, but he was really just a hypnotized pawn of the real villain, Ultron!
Ultron was created by Hank Pym, endowed with a copy of his brainwaves (which, as Hank's size-changing was making him more unstable meant that the robot was likewise mentally unbalanced), Ultron hypnotized Hank into forgetting he created the robot....and went about planning to attack the human race. Ultron failed with his gathering of Avengers villains, but soon returned, creating the Vision to infiltrate the team (getting help from the Grim Reaper so he could give the Vision Wonder Man's brainwave patterns, and getting the body of the original android Human Torch from Fantastic Four foe, the Mad Thinker), Ultron continued his attack against the super-hero team (but was betrayed by the Vision).
Ultron has come back many times over the years, facing the Fantastic Four as well, and looking for a bride of his own (creating the android Jocasta). Ultron was even a part of the Marvel Super Heroes Secret Wars, wherein he was used as a pawn by Dr. Doom...but he returned again as a villain in his own right, being a true menace to the Avengers and the entire human race every time he comes back!