To help him in his fight against Laharl, he hired a bunch of succubi and nekomatas in order to exploit Laharl's fear of anything sexy. However, Laharl managed to overcome this obstacle (with a little "help" from Vyers according to the manga and some help from Etna according to the anime). Maderas got desperate and demanded Etna's aid, only to by betrayed by her and the prinny squad. Since then, he lives a life of servitude to Laharl.
In the game, Maderas' personality and accent is like the one of Bella Lugosi's Dracula, but in the anime his personality is what some people would call "girly and homosexuall".
In one of the games alternate endings, Etna kills Maderas and Flonne (since she tried to stop Etna from getting her revenge due to her own loving and peacefull nature) and it is also speculated that she kills Laharl too in order to become the Overlord and rule the Netherworld.