Black Smoke Shenron is an eternal dragon of pure evil, and is responsible for the creation of the seven deadly Shadow Dragons. The Shadow Dragons are born from the overuse of Dragon Balls and the accumulation of negative energy throughout the series for selfless wishes (with the exception of one born from an extremely selfish desire, and one where the wish that spawned her was a debatable wish in terms of whether it was selfish or selfless in nature) are all initially manifested within Black Smoke Shenron's body. (Source: Dragon Ball Wiki)