Darck is the main antagonist of Dragon Quest Monsters 2 in both Cobi's Journey and Tara's Adventure for Game Boy Color and the Dragon Quest Monsters 2 3DS remake that compiles both. He's a fat, opulent demon, often seen with a lazy stance, that sits in a room filled with treasures. He uses these treasures to trigger negative emotions in humans which enhance his more power. With the global localization since Dragon Quest 8, it is likely that his name will change.
The first time it's encountered is until the end of the game, though he's often referred to on various occasions, he tells them about how obtaining the magical treasures have already began to corrupt their minds and build his power and then the final battle begins.
Darck has a gargantuan, even more opulent form in the 3DS remake called Neo Darck, which he transforms into by eating a shiny orb after he is defeated.