In the popular anime series "Dragon Ball Z", Lord Slug was one of the Ancient Nameks who discovered the key to unlocking a higher consciousness within. Known as Super Nameks, they soon became intoxicated by their new power and began to misuse it. With the power of the Dragon Balls, the people of Namek were able to banish the Super Nameks from their planet for good, but the tyrannical Lord Slug continued conquering planets until he reached to Earth where he used the Dragon Balls to restore his lost youth.

Movie Plot

Lord Slug is the main antagonist only in the Dragonball Z movie #4 - "Lord Slug". He was a very old Namek that had been traveling through space for a number of years after being banished from his homeworld for misusing his powers. Along with his crew of vicious fighters, They traveled from planet to planet wreaking chaos as they went.
Eventually reaching Earth, they attempt to carry out the same destruction there that they had at a countless number of other planets. The way they dominated a planet was to literally change its atmosphere and freezing the planet. By conforming it to their liking, they made it more suitable for themselves and almost unlivable for any other people, including the native occupants of the planet.

Slug turned out to be stiff competition, Slug had aged hundreds of years and was rapidly approaching the end of his life. He managed to gather the dragonballs knowing of their power because of his Namekien heritage. He used the dragonballs to restore his health. Instead of wishing for immortality like other villains did, Slug's wish to have his youth fully restored gave him immense power.

Goku along with his son Gohan and Piccolo squared off against Slug's team and managed to defeat them all. The final battle against Slug was left in the hands of Goku. The battle wasn't going well for Goku until he was discarded by Slug who drew his attention away to Piccolo.
Exploiting a weakness of all Namekians; their extremely sensitive hearing, Gohan's whistling was able to bring Slug to his knees. Piccolo also gave what little strength remained in him to Goku to use to battle against Slug.

Goku was able to take Slug on in an incredibly fierce battle. Slug possessed the ability to transform into an giant version of himself but thanks to Piccolo's quick thinking in using Slug's sensitive hearing against him, Slug was weakened by this. Goku was able to defeat Slug with ease from that point on. The movie ends with Goku flying up into the clouds to deliver the final blow to Slug with a Genki-Dama.
As with other Dragon Ball movies, Slug is considered non-canonical to the Anime or the Manga.