DAGOS is probably the best known of all the Dark Eight, although he is also the least accessible. He does not appear publicly like Zimimar does, but then again, he isn't replaced nearly as frequently. It would not do to replace the Marshall of the Pits, and the commander-in-chief of the Baatorian armies too often.
Even though Zaebos controls promotions in theory, Dagos' underlings frequently bypass his ministry. It is a well-known fact that one of the best ways for any baatezu to rise quickly in rank is by serving Dagos in the Blood War. Bel certainly knows this, and his Lordhood is deeply in debt with Dagos' ministry after serving as the commander of the ground forces.
Dagos is a master of thinking ahead, and the current Dagos (of only two) has always managed to control the flow of war to his advantage towards the other Dark Eight. The only exception is Baalzephon, who he must rely on for supplies. Yet even Baalzephon does not openly oppose Dagos. That's simply not her style, anyway.
The first Dagos was no less successful, and his downfall can hardly be said to have been his own fault. But the uprising of the lesser ranks required a sacrifice, and they got one. Indeed, some say that this uprising was long foreseen by the Dark Eight (or even their mysterious founder), and that a new Dagos was being trained to take over from day one, learning every tactical lesson that the first was exposed to.
But being in a place of such power without actually having the status of a lord is stressful, and Dagos grows increasingly more paranoid - especially towards Corin, the spymaster. He feels that he isn't "in on it", like Furcas and Baalzephon are (they are originals, after all). Thus, he spends increasing amount of time locked up in Malsheem, trying in vain to micromanage his armies and stay safe at the same time.
Dagos always appears in pit fiend form, for maximum effect. He wears his iron mask and spiked armor and never, ever rests.
Pass it on, but don't tell anyone. It's a secret, pass it on.
-The Orroloth
-Isn't it obvious? http://www.hi.is/~orrie/orri-monkey.jpg