The people from the Healing Springs are, as mentioned on that page, a progressive folk, especially for a medieval world. From their senate-based government system to their general desire to advance society, they sort of stand out. And since that sort of thing tends to get your nation raided by zealots led by people perfectly happy with their place in their own societies, it's a good thing the Springs are difficult to just stampede into.
Unfortunately, some people take the concept of "advance society" and append the disheartening phrase "by any means necessary". Marzos is one of those people. Once a contemporary of Salthalus and roughly as wise in his knowledge of magic, Marzos has fancied himself as a sort of explorer of new lands. The problem being, said lands happen to be the unknowable worlds that exist beyond otherworld portals. Historically, said portals have proven incredibly dangerous to open, maintain, and traverse. Well, not traverse. If the portal IS open and stable, it's actually quite easy to traverse. It's more of what can traverse BACK that makes that part dangerous.
Marzos, for his part, is willing to take those risks in the name of exploration and understanding. He just doesn't feel it necessary to ask anyone else nearby who might be at risk ("nearby" being "potentially most of the continent"). That's one vital thing that separates him from the dwarves: They usually have a keener grasp on what puts others in serious danger. At any rate, in the past, since he couldn't be talked out of it and went ahead with conjuring such a portal, some of the more powerful mages from the Springs, among them Salthalus, were forced to stop him. Since people from the Springs do not believe in violent solutions to problems, they imprisoned Marzos with the best binding spells they knew.
And, as it turned out, it worked pretty well for a while. A couple decades or so later, though, he managed to free himself. Nobody's sure how, but what is known is that, after a brief desire for revenge that eventually blew over, he wants to recreate the portal again. He's most likely got the time to do so; it wasn't like binding spells THAT powerful allow your body to age that much in the meantime.
None of this mentions his brother Melzos. Mel went off to become a farmer. That might account for Marzos's frequent use of demon chickens to discourage other mages from interrupting his work.