My knowledge of the animated series gets a little fuzzy here, my knowledge about Piccolo Diamoah comes entirely from reading the manga. And seeing as this saga hasn't been released in America, I had to read the manga in German. But anyway, so there's this guy, a green fellow who wants to be the Guardian of the Earth. But he feels unworthy of the post as long as there is any vestige of evil inside of him. So he purges the evil side and locks it away and becomes the Namek we come to know as Kami. Well this evil side, Piccolo Diamoah, is of course released (remind anyone of the Buu saga?) and he uses the dragonballs to wish for his youth. He then decides the world is his oyster and makes a valliant attempt to take it under his control. As usual, Goku will have none of this and launches an attack at Diamoah, drilling a headbutt right through Diamoah's chest. in his dying breath Diamoah spits up an egg, which later hatches into the Piccolo we know and love from the Dragonball Z series. At this point, Piccolo is still evil. He wants to kill Goku out of vengeance for his paternal unit, and the two of them do battle again. He was such a great character that he was immediately brought back in the first episode of Dragonball Z, he's the first to encounter Raditz (more on him later...) and he's the one who trains Gohan to become the great Sayian that he ended up being. Piccolo's other great semi-villainous appearance was in Dragonball Z Movie 1 "Dead Zone" where he stomached fighting alongside Goku but hurled threats of "you're next" at his advisary. I've got a whole section dedicated to this character, feel free to click on the lefthand sidebar to have a look.