When Hordak and his Horde invaded Eternia and stormed the palace, Hordak broke in and kidnapped one of the King and Queen's twin babies. Man-At-Arms and the Royal Guard captured Hordak's apprentice Skeletor and forced him to divulge the whereabouts of his master. Hordak had retreated to his base of operations Snake Mountain.
When cornered, Hordak opened up a random dimensional portal (later revealed to have led to Etheria) and leaped through with the baby Princess Adora still in his possession. Skeletor remained on Eternia and raised an army of powerful minions of his own and took over as ruler of Snake Mountain. Skeletor's main goal is to conquer the mysterious fortress of Castle Grayskull, from which He-Man draws his powers. If he succeeds, Skeletor would be able to conquer not only Eternia, but the whole universe.
In the original series, Skeletor is the main villain, either appearing in the flesh, or acting thru one of his minions, such as Evil-Lyn or Whiplash. Due to the semi-comedic nature of the show, Skeletor is shown to be arrogant, whiney, childish, and at times incompetent.
Examples of his immature behavior is when he attempted to hijack a circus simply because the owner refused to perform at Snake Mountain, throwing a tantrum at his henchmen's stupidity, and repeating that he isn't a nice man. In one of the post-series minicomics, it's hinted that Skeletor is He-Man's uncle, Prince Keldor.
Sometimes, he would go out of his way to menace She-Ra and attempt to overthrow Hordak out of revenge for being abandoned during Princess Adora's kidnapping.
He-Man and the Masters of the Universe (2002)
In the new continuity of the 2002 animated series, Skeletor's original name is definitely Keldor, his appearance as such is shown and his exploits partially depicted. However it seems unlikely that he is related to Randor in this continuity, as he has Skeletor's blue skin and some other slightly nonhuman features whilst still Keldor. In a he-man.org interview with one of producers of the 2002 series, it is revealed that Keldor is the half brother of Randor; they have different mothers.
Skeletor was formerly a warlord known as Keldor who trained under Hordak. He gathered a small band of warriors to attack the Hall of Wisdom. They encountered resistance from Captain Randor and his officers; Keldor fought Randor personally, wielding two swords with astounding proficiency, but when Randor disarmed him, Keldor threw a vial of acid at him. Randor deflected it with his shield, and the acid splashed on Keldor's face.
Kronis called the retreat, and Evil-Lyn took Keldor to Hordak's sanctuary, where Keldor summoned Hordak to save his life. Keldor agreed to pay whatever price Hordak wished for his life, and Hordak transformed him, stripping the damaged tissues from his skull and dubbing him Skeletor; Keldor's head had been completely stripped of soft tissues, leaving only a floating skull. When Keldor saw his new appearance, he laughed maniacally, the incident perhaps shattering whatever sanity he had left.
Trapped in the Dark Hemisphere by the Mystic Wall, Skeletor designed a machine that would smash it, but it needed the Coridite Crystal as a power source. When Mer-Man retrieved it, Skeletor destroyed the Mystic Wall, and returned to menacing Eternia.
Unlike this previous portrayals, Skeletor is not concerned with Castle Grayskull right away, until a giant fish-monster heads toward it and the Eternian warriors led by Man-At-Arms and He-Man stop it, forcing Skeletor to ponder what could be in that ancient pile of stones worth dying for. At the time, the Heroic Warriors seem ready and willing to let Grayskull fall, and would likely have done so, if not for the urgings of Man-At-Arms, who had been swallowed whole by the monster. Skeletor directly attacks it with his Council of Evil: himself, Count Marzo, Evilseed, the three evil Giants and Webstor. When King Hiss and the Snake Men are freed from the Void, Hiss imprisons Skeletor, having him devoured by a giant snake, but Skeletor escapes after the Masters defeat Hiss.
Despite owing his life to Hordak, Skeletor destroys Hordak's sanctuary to prevent him from returning. At the end of the Second Season, King Hiss revives Serpos, the Serpent God, who had been transformed into Snake Mountain by the Elders; Skeletor and his minions were inside the mountain at that time. Although Serpos is defeated and restored to its Snake Mountain form. If season three of the series been produced, it would have seen Skeletor and He-Man dealing with the Horde invasion and the powerful Hordak, who it was said Skeletor would eventually have defeated.
As with all the Mike Young Productions series' characters, Skeletor's appearance is based on his figure from the Four Horsemen-designed toyline for which the cartoon was produced to promote. Skeletor is the character that received perhaps the least-extensive redesign from his original toy/cartoon version. However, when this new design was then translated into animated form, MYP's artists usually gave him a voluminous cape; something which neither the new toy, nor the original incarnation of the character ever wore. The cape is typically adorned in situations where Skeletor chose to employ powerful magical feats, and may have been influenced by Skeletor's costume in the 1987 Masters of the Universe film. Skeletor is still commonly seen without his cape in the 2002 series while at rest or in combat situations not requiring extensive use of magic. When a later convention-exclusive figure of Keldor was made using the existing Skeletor body, a removable cloth cape was included. As the figure came with three swappable heads including his Keldor face; his burning, acid-splashed visage; and his final Skeletor head, this figure could thus be configured into a "show-accurate" caped Skeletor.
The New Adventures of He-Man
In the controversial semi-sequel series, Skeletor is still the main villain, but is presented as being more mature and villainous than in the Filmation series, and has a very morbid sense of humor, often spouting out one liners and jokes. In this series, Skeletor follows He-Man to the world of Primus and joins forces with the Mutants, pretending to follow the orders of the idiotic Flogg, while manipulating him all the way.
In episode six, Skeletor is transformed into a more powerful version of himself via an evil crystal and stays that way until the final episode. In episode 65, Skeletor is finally defeated when He-Man sends him and his lover Crita rocketing thru space. In the final scene, Skeletor vows that he'll return and gives out a chilling laugh.
Live Action Film
Skeletor also appeared in the live action film Masters Of The Universe and was obsessed with becoming a god via stealing the Key Of The Universe - however his plans were opposed by He-Man and his group, who went as far as opening a portal to Earth in order to evade him: Skeletor wasn't so easily shaken however as he sent several henchmen after them (most of them well-known He-Man villains such as Beast-Man). When his servants failed him Skeletor arrived on Earth himself via a huge floating throne, accompanied by an entire army and forced He-Men into becoming his slave with the threat of killing his friends should he resist.
Skeletor then took the Key Of The Universe and with it became a god, transforming into a golden vaguely Aztec-inspired super-being as a result - however He-Man's friends managed to come to his aid and when Skeletor ordered his men to kill them He-Man became enraged and the two enemies engaged in a climatic battle, although he put up a good fight Skeletor wasn't a match for He-Man and ultimately lost the battle: transforming back to his usual form after his godlike energy was dispersed.
He-Man attempted to show mercy to his old foe by informing him that the battle was over but Skeletor replied with a taunting "yes... for you!" before attempting to kill He-Man again, only to plunge to his death down a pit. Only for him to live and come out of the water and say "I'll be back!"