Each of them has developed his own theme (or, more accurately) obsession. The first Elder we meet is in Avengers #28. This is the Collector--who's theme is, well, collecting stuff. Including living beings.
At the time, the Collector wasn't identified as a member of a group. Neither was the second Elder we meet. We learn about the Elders in a retcon from Avengers #178 (August 1978). It was one of those inspired retcons that fits perfectly into what we already know and enriches the history of those characters involved. It opens the door for having an array of characters with different themes who can be called upon to appear whenever a story requires it.
Anyway, we meet our second Elder in Avengers #69 (October 1969). This is the first of a three-part arc written by Roy Thomas, with strong art by Sal Bucsema. It's a fun story, structured around an idea that gives the Avengers (both as a group and as individuals) to get into fights with a nifty array of opponents.
It begins with the Avengers visiting a badly injured Tony Stark in the hospital (Tony having been hurt over in his own book). Kang's old henchman, the Growing Man, attacks and kidnaps Tony. The Avengers raise violent objection to this, only to have everyone involved zapped into the future to face off against Kang.
Kidnapping Tony is a ploy to get the Avengers here. (Tony, by the way, has served his purpose by now. He's zapped back to the hospital so that he can get operated on in his own book, then rejoin the Avengers as Iron Man in the next issue.)
The deal is this: The Grandmaster, who's thing is arranging fights and betting on the outcome, offers Kang the opportunity to pit champions of his (Kang's) choosing against Grandmaster's guys. If Kang wins, he'll get the power to revive his comatose girlfriend. If Kang loses, the Grandmaster destroys the Earth. Despite being annoyed with Kang, the Avengers agree to all this in order to save the world.
It's a great plot device--simple, but justified by the Grandmaster's chosen method of passing the time for all eternity, AND giving Thomas and Buscema a logical excuse for choreographing a lot of cool fight scenes.
This brings us to Avengers #70 and the introduction of yet another cool set of characters. Cap, Iron Man, Thor and Goliath (Clint Barton using Hank Pym's growth serum) must individually face off against members of the Squadron Sinister, each of whom is an obvious expy of a member of DC's Justice League. Hyperion is Superman (though he comes from a micro-universe destroyed in an atom smasher), Dr. Spectrum is Green Lantern, The Whizzer is Flash and Nighthawk is Batman.
The ensuing fight scenes are all exciting and well-choreographed. Cap beats Nighthawk in the first match-off. I love that. Though I am a huge fan of both Captain America and Batman, I always see Cap winning a fight between the two. So Cap taking down an ersatz-Batman is very satisfying for me.
Iron Man and Thor take down Spectrum and Hyperion, though it is possible to complain about the Deus Ex Machina that ends Thor's fight. He essentially asks Odin for a convenient new super power he never had before (and probably has never had since), allowing him to shrink Hyperion down and encase him in glazed sand. It's as if the book was suddenly being edited by Mort Weisinger.
Things go awry during the Goliath/Whizzer fight, which is taking place in London. The Black Knight is nearby and sees the battle. From his point-of-view, it's perfectly reasonable for him to jump in and help the good guy. He does so, but this nullifies the entire contest. The Avengers are all returned to the 40th Century, with Goliath still holding the Knight's sword. That's a plot point, since the Black Knight will use his mystical connection to the sword to time travel to the Avengers in the next issue.
Avengers #71 allows Roy Thomas to indulge in his love of Golden Age heroes. The re-vamped contest now involves Black Panther, Vision and Yellowjacket being zapped back to occupied Paris in 1941, where they have to face off against pre-frozen Captain America, Namor and the original Human Torch. Being a fan of the Golden Age as well (though no one can match Roy Thomas in his Golden Age fervor), I love this part.
By the way, when Thomas wrote Invaders Annual #1 in 1977, we got to see this fight again from the Invaders' point-of-view. Thomas did have to contrive a reason for giving Cap back his triangular shield (which he hadn't been using in the Invaders), but otherwise the two versions of the story mesh nicely.
Using several clever tactics, the Avengers win the fight. Meanwhile, the Black Knight has arrived in the 40th Century. This is important, because Kang isn't able to overcome his hatred of the Avengers even when the life of his true love is at stake. He demands the power to kill the Avengers as his prize rather than the power to revive the girl.
Unfortunately for Kang, the Black Knight isn't yet an Avenger--AND the Grandmaster is one of those "exact words" guys. So the Knight is able to knock out Kang. One can argue the ending wraps things up too quickly and conveniently--even with the element of surprise, Dane Whitman shouldn't have the raw power to take down Kang with one shot. Also, I'm not quite sure how all this cancelled out Grandmaster's deal with Kang and allows the Elder to revive the Avengers and send everyone home.
But these are arguable points. Overall the story does exactly what it's supposed to do. It constructs a plot that fits nicely into Comic Book Logic and builds on this to give us a series of really nifty fight scenes. Kang's "give-life-or-give-death" choice provides some emotional impact and irony. It's always nice to have emotional impact and irony, but gosh-darn-it, sometimes it's even nicer to see Captain America curb-stomp fake-Batman!