The Shadow Dragons[1] (邪悪龍, Jāku Ryū, lit. "Evil Dragons") are evil beings created by the overuse and built-up negative energies of the Dragon Balls over the series, and are the final villains in Dragon Ball GT.
The avatars taken over by the Shadow Dragons in Dragon Ball Heroes: Victory Mission are referred to collectively as the Disciples of Evil.
Concept
The Shadow Dragons were all part of one original dragon.Overview
Dragon Ball GT
The Shadow Dragons are a direct consequence of the numerous uses of the Dragon Balls over the course of the Dragon Ball series. Every time a wish was made with the balls, negative energy was released along with the positive energy. In response, the Dragon Balls would absorb the negative energy and disperse it harmlessly over many years.
The Shadow Dragons being unleashed
The fight against the Shadow Dragons makes up the events of the Shadow Dragon Saga. The battle against the evil dragons can be considered the closing segment of the series (finding the Dragon Balls was the beginning) that teaches the Earth to stop relying on the Dragon Balls to solve their problems. It can also be seen as the heroes' ultimate challenge: when the Dragon Balls finally turn against the Z Fighters.

Omega and Nuova Shenron
Of the seven, only Nuova, Eis and Syn Shenron have martial arts skills which pose a serious challenge to Goku; the other four rely more on trickery and their elemental powers. Nuova Shenron is believed to have the least negative energy of the Shadow Dragons, due to his morals and the wish that creates him. While most of the other dragons were made by very selfless wishes (or in the case of Oolong's underwear wish, which created Oceanus, a wish in the gray area), Nuova is created by a very selfish wish made by King Piccolo, who wishes for his youth and power to be restored.
Dragon Ball Heroes: Victory Mission

The Shadow Dragons are released once more
Black Smoke Shenron fights against the heroes using Shadow GT Goku after Dr. Auto summons him. However, Black Smoke Shenron is defeated when Adult Gotenks arrives and destroys him using his Burning Kamehameha.
http://dragonball.wikia.com/wiki/Shadow_Dragons